initial Reconfigurator developer guide with SP update instructions

[davepacheco]

Here's a first draft of some developer documentation for Reconfigurator. The main motivation for now is documenting how to do SP updates using the lower-level tools like reconfigurator-sp-updater. It was tricky to figure out how to structure this because the process is (1) long, (2) similar but not the same in different kinds of test environments, (3) made up of pieces that can also stand on their own as useful steps. I wanted steps to be individually linkable but also have a coherent summary of all the things you need to do if you're doing them all together.

There's enormous room for added scope and improvement, but I think this is quite a lot better than what we've got now (which is nothing) so I'll plan to land this in short order unless folks feel like this is somehow a step in the wrong direction. I'd love to have help fleshing the rest of it out once this first version lands. (There are a bunch of suggestions in the comments at the top of the file!)