Document repository rules (closes #1245)

docs/contributing.md

@@ -190,6 +190,19 @@ revision file which can be created with the following command:
 
 You then need to write the migration steps yourself.
 
+## Repository rules
+
+- Please, try to keep it to **one pull request per feature:** if you want to do more
+  than one thing, send multiple pull requests. It will be easier for us to review and
+  merge.
+- **Document your code:** if you add a new feature, please document it in the
+- All the people working on this project do it on their spare time. So please, be
+  patient if we don't answer right away.
+- We try to have two maintainers review the pull requests before merging it. So please,
+  be patient if we don't merge it right away. After one week, only one maintainer approval
+  is required.
+
+
 ## How to build the documentation ?
 
 The documentation is using