Sync CONTRIBUTING.md text with engine.git (#8004)
Sync CONTRIBUTING.md text with engine.git (#8004)

* Sync CONTRIBUTING.md text with engine.git

Turns out we had very similar text in engine.git.  This patch makes the text the same by adopting the text in engine.git.

* Update CONTRIBUTING.md

cbracken/flutter,flutter/flutter,flutter/flutter,turnidge/flutter,Hixie/flutter,turnidge/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,turnidge/flutter,turnidge/flutter,turnidge/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter