Initial user and instrument docs.

[AustinSanders]

Partially addresses #386

As it stands, this documentation includes:

1. instructions for generating and validating an ISD, including how to handle situations in which ale doesn't find a kernel set, i.e. getting kernels from a spiceinit'd cub
2. processing instructions for WAC
3. a (very) brief summary of each sensor for which ALE drivers exist. These could use additional information, but I wasn't sure what information was desirable in this context
4. a few processing examples. I'd like to get an example for each supported driver. I'm running through knoten + other examples, but I'm going through the process of making sure they actually work + trying to sniff out code rot.

[jessemapel]

I think this is ready to get merged. It's not a complete set of documentation, but it's a whole lot and a very good base to build off of.