add CONTRIBUTING file

Change-Id: I52a988bc02021c2d90c586801fbe811e602196d7
Tuleap: #451

CONTRIBUTING.md

+# How to contribute
+
+Ring loves external contributions. But we do not use Github PR system.
+Instead we host a public gerrit server.
+
+    https://gerrit-ring.savoirfairelinux.com
+
+More documentation on Gerrit:
+
+    https://www.gerritcodereview.com/
+
+In each Ring modules there is a .gitreview file. It contains all the
+necessary info to send a patchset to our gerrit.
+
+After you commited your changes (one or multiple commits) you only
+need to submit them with:
+
+    git-review
+
+## Making Changes
+
+* [Optionnal] Create a ticket in our Tuleap bug tracker https://tuleap.ring.cx/projects/ring
+* Make commits of logical units.
+* Check for unnecessary whitespace with `git diff --check` before committing.
+* Make sure your commit messages are in the proper format
+    - 50 chars title
+    - 80 chars message width