CONTRIBUTING.md: Initial release

As per issue #2623, it is important to define the commit guidelines.
This commit is about adding a first version of it.

Fixes: #2653
Signed-off-by: Erwan Velu <erwan@redhat.com>

CONTRIBUTING.md

@@ -0,0 +1,33 @@
+Contributing to ceph-ansible
+==============================
+
+1. Follow the [commit guidelines](#commit-guidelines)
+
+
+Commit guidelines
+-----------------
+- All commits should have a subject and a body
+- The commit subject should briefly describe what the commit changes
+- The commit body should describe the problem addressed and the chosen solution
+  - What was the problem and solution? Why that solution? Were there alternative ideas?
+- Wrap commit subjects and bodies to 80 characters
+- Sign-off your commits
+- Add a best-effort scope designation to commit subjects. This could be a directory name, file name,
+  or the name of a logical grouping of code. Examples:
+  - library: add a placeholder module for the validate action plugin
+  - site.yml: combine validate play with fact gathering play
+  - rhcs: bump version to 3.0 for stable 3.1
+- Commits linked with an issue should trace them with :
+  - Fixes: #2653
+
+Suggested reading: https://chris.beams.io/posts/git-commit/
+
+
+CI
+-----
+
+### Jenkins
+We use Jenkins to run several tests on each pull request.
+
+If you don't want to run a build for a particular pull request, because all you are changing is the
+README for example, add the text `[skip ci]` to the PR title.