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/tools or https://github.com/golang/tools
figure out how to get started with development.
Change-Id: I778202d9759df5cf79e86b8524046e108aa3db3d
Reviewed-on: https://go-review.googlesource.com/49852
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
