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://golang.org/cl/74370043
1 file changed