Add README.

README

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+*TODO(dchest): Add example images and audio.*
+
+Package captcha
+=====================
+
+	import "github.com/dchest/captcha"
+
+Package captcha implements generation and verification of image and audio
+CAPTCHAs.
+
+
+Constants
+---------
+
+	const (
+		// Standard number of digits in captcha.
+		StdLength = 6
+		// The number of captchas created that triggers garbage collection used
+		// by default store.
+		CollectNum = 100
+		// Expiration time of captchas used by default store.
+		Expiration = 10 * 60 // 10 minutes
+
+	)
+
+
+	const (
+	    // Standard width and height of a captcha image.
+	    StdWidth  = 300
+	    StdHeight = 80
+	)
+
+
+
+Variables
+---------
+
+	var ErrNotFound = os.NewError("captcha with the given id not found")
+
+
+
+Functions
+---------
+
+### func Collect
+
+	func Collect()
+	
+Collect deletes expired or used captchas from the internal storage. It is
+called automatically by New function every CollectNum generated captchas,
+but still exported to enable freeing memory manually if needed.
+
+Collection is launched in a new goroutine.
+
+### func New
+
+	func New(length int) (id string)
+	
+New creates a new captcha of the given length, saves it in the internal
+storage, and returns its id.
+
+### func RandomDigits
+
+	func RandomDigits(length int) []byte
+	
+RandomDigits returns a byte slice of the given length containing random
+digits in range 0-9.
+
+### func Reload
+
+	func Reload(id string) bool
+	
+Reload generates and remembers new digits for the given captcha id.  This
+function returns false if there is no captcha with the given id.
+
+After calling this function, the image or audio presented to a user must be
+refreshed to show the new captcha representation (WriteImage and WriteAudio
+will write the new one).
+
+### func Server
+
+	func Server(w, h int) http.Handler
+	
+Server returns a handler that serves HTTP requests with image or
+audio representations of captchas. Image dimensions are accepted as
+arguments. The server decides which captcha to serve based on the last URL
+path component: file name part must contain a captcha id, file extension —
+its format (PNG or WAV).
+
+For example, for file name "B9QTvDV1RXbVJ3Ac.png" it serves an image captcha
+with id "B9QTvDV1RXbVJ3Ac", and for "B9QTvDV1RXbVJ3Ac.wav" it serves the
+same captcha in audio format.
+
+To serve an audio captcha as downloadable file, append "?get" to URL.
+
+To reload captcha (get a different solution for the same captcha id), append
+"?reload=x" to URL, where x may be anything (for example, current time or a
+random number to make browsers refetch an image instead of loading it from
+cache).
+
+### func SetCustomStore
+
+	func SetCustomStore(s Store)
+	
+SetCustomStore sets custom storage for captchas, replacing the default
+memory store. This function must be called before generating any captchas.
+
+### func Verify
+
+	func Verify(id string, digits []byte) bool
+	
+Verify returns true if the given digits are the ones that were used to
+create the given captcha id.
+
+The function deletes the captcha with the given id from the internal
+storage, so that the same captcha can't be verified anymore.
+
+### func VerifyString
+
+	func VerifyString(id string, digits string) bool
+	
+VerifyString is like Verify, but accepts a string of digits.  It removes
+spaces and commas from the string, but any other characters, apart from
+digits and listed above, will cause the function to return false.
+
+### func WriteAudio
+
+	func WriteAudio(w io.Writer, id string) os.Error
+	
+WriteAudio writes WAV-encoded audio representation of the captcha with the
+given id.
+
+### func WriteImage
+
+	func WriteImage(w io.Writer, id string, width, height int) os.Error
+	
+WriteImage writes PNG-encoded image representation of the captcha with the
+given id. The image will have the given width and height.
+
+
+Types
+-----
+
+	type Audio struct {
+    // contains unexported fields
+}
+	
+
+### func NewAudio
+
+	func NewAudio(digits []byte) *Audio
+	
+NewImage returns a new audio captcha with the given digits, where each digit
+must be in range 0-9.
+
+### func (*Audio) EncodedLen
+
+	func (a *Audio) EncodedLen() int
+	
+EncodedLen returns the length of WAV-encoded audio captcha.
+
+### func (*Audio) WriteTo
+
+	func (a *Audio) WriteTo(w io.Writer) (n int64, err os.Error)
+	
+WriteTo writes captcha audio in WAVE format into the given io.Writer, and
+returns the number of bytes written and an error if any.
+
+	type Image struct {
+	    *image.NRGBA
+	    // contains unexported fields
+	}
+	
+
+### func NewImage
+
+	func NewImage(digits []byte, width, height int) *Image
+	
+NewImage returns a new captcha image of the given width and height with the
+given digits, where each digit must be in range 0-9.
+
+### func (*Image) WriteTo
+
+	func (img *Image) WriteTo(w io.Writer) (int64, os.Error)
+	
+WriteTo writes captcha image in PNG format into the given writer.
+
+	type Store interface {
+	    // Set sets the digits for the captcha id.
+	    Set(id string, digits []byte)
+
+	    // Get returns stored digits for the captcha id. Clear indicates
+	    // whether the captcha must be deleted from the store.
+	    Get(id string, clear bool) (digits []byte)
+
+	    // Collect deletes expired captchas from the store.  For custom stores
+	    // this method is not called automatically, it is only wired to the
+	    // package's Collect function.  Custom stores must implement their own
+	    // procedure for calling Collect, for example, in Set method.
+	    Collect()
+	}
+	
+An object implementing Store interface can be registered with SetCustomStore
+function to handle storage and retrieval of captcha ids and solutions for
+them, replacing the default memory store.
+
+### func NewMemoryStore
+
+	func NewMemoryStore(collectNum int, expiration int64) Store
+	
+NewMemoryStore returns a new standard memory store for captchas with the
+given collection threshold and expiration time in seconds. The returned
+store must be registered with SetCustomStore to replace the default one.
+
+
+Bugs
+----
+
+[Not our bug] Google Chrome 10 plays unsigned 8-bit PCM WAVE
+audio on Mac with horrible distortions.  Issue:
+http://code.google.com/p/chromium/issues/detail?id=70730.
+This has been fixed, and version 12 will play them properly.
+
+While Image conforms to io.WriterTo interface, its WriteTo
+method returns 0 instead of the actual bytes written because png.Encode
+doesn't report this.