Add a doc on how to migrate from SVN to gitlab.
Clearer what to contribute.
Git is a distributed vcs.
Add links to the migration doc, make it clear import is only for git repos.
Merge branch 'migrating_from_svn_doc'

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Conflicts:
	doc/workflow/README.md
Merge branch 'master' of github.com:gitlabhq/gitlabhq