doc/contributing.rdoc: allow/encourage other git hosts

git is a distributed system, so make it clear to potential contributors
that it does not and should not matter where their repositories are
hosted.

Furthermore, it should be possible to contribute to Open Source projects
by relying only on Open Source tools, protocols and APIs.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44729 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-01-29

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44730 b2dd03c8-39d4-4d8f-98ff-823fe69b080e