Docs: add GitHub PR template and link to contributing docs
Docs: add GitHub PR template and link to contributing docs

A number of users don't realize that GitHub is a read-only mirror
and that code should flow through gerrit.
Make it explicit.

Change-Id: I4cabc924be637f5492ff711d320b1e0030d7ebd3
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto