Update 'contributing to cegui' to reflect preference for Hg pull requests.
Update 'contributing to cegui' to reflect preference for Hg pull requests.

--HG--
branch : v0-8

cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two