doc: Revise Contribution Guidelines.
doc: Revise Contribution Guidelines.

Smooth out the setup process for new contributors.

* Remove references $GOROOT (often not defined).
* Add a note for contributing to subrepositories.
* Emphasize that hg mail also uploads the latest copy.

LGTM=adg
R=golang-codereviews, iant, adg
CC=golang-codereviews
https://codereview.appspot.com/74370043

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang