Import rake task documentation in separate file.

doc/raketasks/import.md

+### Import bare repositories into GitLab project instance
+
+Notes:
+
+* project owner will be a first admin
+* groups will be created as needed
+* group owner will be the first admin
+* existing projects will be skipped
+
+How to use:
+
+1. copy your bare repos under git repos_path (see `config/gitlab.yml` gitlab_shell -> repos_path)
+2. run the command below
+
+```
+bundle exec rake gitlab:import:repos RAILS_ENV=production
+```
+
+Example output:
+
+```
+Processing abcd.git
+ * Created abcd (abcd.git)
+Processing group/xyz.git
+ * Created Group group (2)
+ * Created xyz (group/xyz.git)
+[...]
+```