Document capabilities

[rplzzz]

Each capability defined in the system is supposed to publish documentation that describes everything that other components will need to use the data (format, units, etc.) Right now most of that documentation exists only in our heads; before too long we will need to get this written down.

First we need to decide where we plan to capture this information. One possibility is to create a wiki for the repository. Another is to include some supplementary documentation that we will eventually publish on readthedocs.org. Whatever we do, we need to think about what happens if other groups adopt Cassandra for their models. How will they manage the documentation for their components' capabilities?