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

Change-Id: Iafdbec407bd6f61459aabe6e2c4703132d038b69
Reviewed-on: https://go-review.googlesource.com/49872


Reviewed-by: Kevin Burke <[email protected]>