Add how-to documentation

This is only a documentation change, safe to release to both Groovy and Hydro.

This partially fixes ros/catkin#410. The unit test sections still need to be written, but I am still learning how to do that myself, and it is difficult enough to significantly delay completion. 

So, I recommend releasing the other sections, which have already undergone considerable review on **ros-sig-buildsystem**. I see many questions on **answers.ros.org** that could benefit from this information.