Add Pull Request documentation

CONTRIBUTING.md

@@ -3,7 +3,9 @@
 You can contribute in a few different ways:
 
 * Submit issues through our [issue tracker](https://github.com/bbva/qed/issues) on Github.
+* If you wish to make code changes please check out [Our Guidelines](docs/pull_requests.md) about **Pull Requests** 
+    and [GitHub Forks / Pull requests model](https://help.github.com/articles/fork-a-repo/):
 * If you wish to make code changes, or contribute something new, please follow the
-[GitHub Forks / Pull requests model](https://help.github.com/articles/fork-a-repo/):
+
 fork the repo, make the change and propose it back by submitting a pull request.
 * Useful notes used by the team are in [docs/development.md](docs/development.md) file 

docs/pull_requests.md

@@ -0,0 +1,38 @@
+# Why this guide
+
+We follow a *work agreement* to provide a linear history, with at most one branch in parallel.
+We also require all commits in master to pass the tests.
+
+For that to happen this guide enable you get your pull request ready for merge into the **master branch**.
+
+# How to make a Pull Request to QED
+
+TL;DR: always rebase to master before attempting to merge into master.
+```bash
+
+# download repo
+git clone [email protected]:bbva/qed.git
+
+# create your fork
+hub fork
+
+# add main organization
+git remote add -f bbva [email protected]:bbva/qed.git
+
+# create branch
+git checkout -b <my-cool-branch>
+
+# ... do some groovy changes
+git commit -am "msg"
+
+# create PR
+$ hub pull-request --base bbva:master
+
+# wait for feedback (surely master will advance)
+
+# once it's approved and ready to merge rebase and resolve conflicts.
+git rebase master
+
+# check again tests are ok, and then merge
+hub merge https://github.com/bbva/pr/pull/13
+```