update to CONTRIBUTING.md related to new update-packages command
update to CONTRIBUTING.md related to new update-packages command

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