Add branches naming convention to contribution file (Activiti#1268)

CONTRIBUTING.md

@@ -4,5 +4,15 @@ This project is a community driven project, so get in touch if you want to parti
 Unit tests
 --------------------
 Activiti uses now [AssertJ](http://joel-costigliola.github.io/assertj/assertj-core-features-highlight.html) 
-as alternative to default JUnit assertions. This makes your tests more readable and provides clearer failure messages. 
-Please, make sure that you are using `AssertJ` assertions for every new test.
+as alternative to default JUnit assertions. This makes tests more readable and provides clearer failure messages. 
+Please, make sure that you are using `AssertJ` assertions for every new test.
+I.e.
+* `assertThat(myBoolean).isTrue();` instead of `assertTrue(myBoolean);`
+* `assertThat(actual).isEqualTo(expected);` instead of `assertEquals(expected, actual);`
+* `assertThat(myList).containsExactly("a", "b", "c");` instead of `assertTrue(myList.contains("a", "b", "c"));`
+
+Branches naming convention
+--------------------------
+If you are creating branches directly in the [main repository](https://github.com/Activiti/Activiti), this is 
+ the naming convention to be used for branches:
+ - `username-issueNumber-anythingYouWant`