README: add improvements

Move the README to README.md so Gerrit can render it; currently
Gerrit only renders files named exactly "README.md" (for example at
https://go.googlesource.com/go).

Add more links to the README explaining how to file issues,
how to submit code changes, where to download the code to and
how to get it. Hopefully this should help people who go to
https://go.googlesource.com/crypto or https://github.com/golang/crypto
figure out how to get started with development.

Change-Id: I5406d0e6a562ef4a4146b66edc53a6fba7a9c3ee
Reviewed-on: https://go-review.googlesource.com/49811
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

README

@@ -1,3 +0,0 @@
-This repository holds supplementary Go cryptography libraries.
-
-To submit changes to this repository, see https://golang.org/doc/contribute.html.

README.md

@@ -0,0 +1,21 @@
+# Go Cryptography
+
+This repository holds supplementary Go cryptography libraries.
+
+## Download/Install
+
+The easiest way to install is to run `go get -u golang.org/x/crypto/...`. You
+can also manually git clone the repository to `$GOPATH/src/golang.org/x/crypto`.
+
+## Report Issues / Send Patches
+
+This repository uses Gerrit for code changes. To learn how to submit changes to
+this repository, see https://golang.org/doc/contribute.html.
+
+The main issue tracker for the crypto repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/crypto:" in the
+subject line, so it is easy to find.
+
+Note that contributions to the cryptography package receive additional scrutiny
+due to their sensitive nature. Patches may take longer than normal to receive
+feedback.