cheat sheet for common patterns

patterns that could be included:
* general idea of 2 targets per data file, one for git and one for local and/or cache
* 3-target pattern for create, push, retrieve a data file
* 2-target pattern for create+push, retrieve a data file
* rolling your own indicator file with sc_indicate - when to do, how to get a file hash in there instead of a timestamp
* use of indicator files to document completion of many steps rather than creation of a specific data file
* use of symbolic targets (with or without indicator files), with notes about when these update properly and when they don't