add CONTRIBUTING.md for github pull request

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11943 e93f8b46-1217-0410-a6f0-8f06a7374b81
svn propset svn:eol-style native CONTRIBUTING.md


git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11944 e93f8b46-1217-0410-a6f0-8f06a7374b81