doc: update contribute.html

doc: update contribute.html

Fixes #4582.

* mentioned hg upload.
* added section on hg file.
* added small mention about being inside $GOROOT.
* added hg revert @NNNN
* reorganise the hg submit section for the common case of a non committer.
* made the Copyright section h2
* added note about leaving copyright years unchanged.

R=golang-dev, metanata4, shivakumar.gn, minux.ma, adg, shanemhansen
CC=golang-dev, metanata4
https://golang.org/cl/7278047