Adds some documentation and README

README.md

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+# CORS gin's middleware
+Gin middleware/handler to enable CORS support.
+
+## Usage
+
+###Canonical example:
+
+```go
+package main
+
+import (
+	"time"
+
+	"github.com/gin-contrib/cors"
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+	router := gin.Default()
+	// CORS for https://foo.com and https://github.com origins, allowing:
+	// - PUT and PATCH methods
+	// - Origin header
+	// - Credentials share
+	// - Preflight requests cached for 12 hours
+	router.Use(cors.New(cors.Config{
+		AllowOrigins:     []string{"https://foo.com"},
+		AllowMethods:     []string{"PUT", "PATCH"},
+		AllowHeaders:     []string{"Origin"},
+		ExposeHeaders:    []string{"Content-Length"},
+		AllowCredentials: true,
+		AllowOriginFunc: func(origin string) bool {
+			return origin == "https://github.com"
+		},
+		MaxAge: 12 * time.Hour,
+	}))
+	router.Run()
+}
+```
+
+###Using DefaultConfig as start point
+
+```go
+func main() {
+    router := gin.Default()
+    // - No origin allowed by default
+    // - GET,POST, PUT, HEAD methods
+    // - Credentials share disabled
+    // - Preflight requests cached for 12 hours
+    config := cors.DefaultConfig()
+    config.AllowOrigins = []string{"http://google.com"}
+    config.AddAllowOrigins("http://facebook.com")
+    // config.AllowOrigins == []string{"http://google.com", "http://facebook.com"}
+
+    router.Use(cors.New(config))
+    router.Run()
+}
+```
+
+###Default() allows all origins
+
+```go
+    router := gin.Default()
+    // same as
+    // config := cors.DefaultConfig()
+    // config.AllowAllOrigins = true
+    // router.Use(cors.Default())
+    router.Use(cors.Default())
+    router.Run()
+```
+
+
+

examples/example.go

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+package main
+
+import (
+	"time"
+
+	"github.com/gin-contrib/cors"
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+	router := gin.Default()
+	// CORS for https://foo.com and https://github.com origins, allowing:
+	// - PUT and PATCH methods
+	// - Origin header
+	// - Credentials share
+	// - Preflight requests cached for 12 hours
+	router.Use(cors.New(cors.Config{
+		AllowOrigins:     []string{"https://foo.com"},
+		AllowMethods:     []string{"PUT", "PATCH"},
+		AllowHeaders:     []string{"Origin"},
+		ExposeHeaders:    []string{"Content-Length"},
+		AllowCredentials: true,
+		AllowOriginFunc: func(origin string) bool {
+			return origin == "https://github.com"
+		},
+		MaxAge: 12 * time.Hour,
+	}))
+	router.Run()
+}