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 <[email protected]>