add CONTRIBUTING.md
add CONTRIBUTING.md

serves as a brief introduction and adds a link to the gerrit
instructions on webmproject.org.

Bug: webm:1669
Change-Id: If1d483eb48e2edcda8c51e66bdd1a86b7c35b986
(cherry picked from commit 220b00dd0d624a24a081fa60e35b024357c506ad)

ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx