Recommend -j1 when running tests.

Closes #409.

CONTRIBUTING.md

@@ -15,3 +15,12 @@ We follow the [C++ ROS style guidelines](http://ros.org/wiki/CppStyleGuide) and
 conventions as closely as possible. However, because the plugins inherit from Gazebo 
 classes and Gazebo follows a very different formatting standard, there are a few 
 exceptions where Gazebo's function names do not comply to the ROS guidelines.
+
+## Tests
+
+We encourage developers to include tests in their pull requests when possible,
+both for bug fixes and new features.
+Examples of tests are in the `gazebo_plugins/test*` folders.
+Currently the tests must not be run in parallel (see issue 
+[issue 409](https://github.com/ros-simulation/gazebo_ros_pkgs/issues/409))
+so it is recommented to use the `-j1` argument to `catkin run_tests`.