Update readme

README.md

@@ -4,4 +4,13 @@ This plugins aims to filter assignable groups in ticket by selected category.
 You can define a level to your glpi groups.
 Then, for each of your ticket's category, you can define which group will be viewed or not.
 
-![Define filters](https://raw.githubusercontent.com/PluginsGLPI/itilcategorygroups/master/screenshots/category_config.png)
+![Define filters](https://raw.githubusercontent.com/PluginsGLPI/itilcategorygroups/master/screenshots/category_config.png)
+
+Contributing
+------------
+
+* Open a ticket for each bug/feature so it can be discussed
+* Follow [development guidelines](http://glpi-developer-documentation.readthedocs.io/en/latest/plugins.html)
+* Refer to [GitFlow](http://git-flow.readthedocs.io/) process for branching
+* Work on a new branch on your own fork
+* Open a PR that will be reviewed by a developer