Document mdsal-it-base and mdsal-it-parent

mdsal-it-base and mdsal-it-parent provide substrate perform full-blown integration testing of components using MD-SAL. These are currently hosted in the Controller project, but we want to migrate them to MD-SAL (and hence ditch CDS dependency, etc.).

In order to do that, though, we need to document best practices on how to use this substrate and what the caveats are.