add contributing.md

Change-Id: Ia3284a90dfbbaaf655facd885a8ef13858bdb2c9

documentation/CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contributing
+
+Prometheus uses Gerrit to manage reviews of pull-requests, and then
+Gerrit replicates its master branch to GitHub. In order to contribute to
+Prometheus, you must use Gerrit.
+
+## Setup
+
+1. Sign in at http://review.prometheus.io/
+2. Set a username and upload an SSH pubkey for git ssh access. `cat
+   ~/.ssh/id_rsa.pub | pbcopy` will copy your public key to your
+   clipboard so you can paste it.
+3. Clone the repo: `git clone http://review.prometheus.io/prometheus`
+4. Add your user-specific remote that you will push your changes to:
+   `git remote add <your-remote-name> ssh://<username>@review.prometheus.io:29418/prometheus`
+4. Add Change-Id commit hook: "curl -o .git/hooks/commit-msg http://review.prometheus.io/tools/hooks/commit-msg"
+6. Make the file executable: `chmod u+x .git/hooks/commit-msg`
+7. Commit any local changes to git, then:
+8. `git push <your-remote-name> HEAD:refs/for/master`
+9. Assign reviewer for change at http://review.prometheus.io/
+
+That's all!