Document maintenance

[pleia2]

The documentation update referenced in #2 is helpful, but there are a few things related to on-going maintenance of the production deployment that we should also document:

• Updating the git submodule here in the -deploy repository
• Updating source files (should we periodically pull from the -data repository?)
• Updating source configuration (use config_build.py after updating source files?)
• Debugging failures after the trigger runs (for instance: it seems like the cache file was updated and the application restarted, but changes in software-discovery-tool since the last update were not reflected in the files on the system)