New plugin for git-repo (https://code.google.com/p/git-repo/)

Signed-off-by: Gaetan Semet <gaetan@xeberon.net>

plugins/repo/README.md

+## repo
+**Maintainer:** [Stibbons](https://github.com/Stibbons)
+
+This plugin mainly add support automatic completion for the repo command line tool:
+http://code.google.com/p/git-repo/
+
+* `r` aliases `repo`

plugins/repo/repo.plugin.zsh

+# Aliases
+alias r='repo'