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/net or https://github.com/golang/net
figure out how to get started with development.

Change-Id: I4eb54f2b210e4f028c9f00955ff19b55643315e6
Reviewed-on: https://go-review.googlesource.com/49850Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>