add more desi.json documentation

[sbailey]

Motivated by #2, add more documentation about how to add a new entry to etc/desi.json, e.g.

1. naming constraints on the tar files, and whether those are best-practices guidelines or actually required for desiBackup and restore to work properly.
2. worked examples for using regex groups to map N>>1 directories into N>>1 tar files, and how much one should worry about the resulting size of the tar file (and how to tell).
3. worked examples for how to map the files in a directory into a tarball without accidentally mapping files in subdirectories.

Since json files don't allow comments, the existing etc/desi.json is only partially useful for deriving these patterns.

[weaverba137]

For my own notes, this documentation is sufficiently important that it should be added to the hpsspy package, and it will be visible at http://hpsspy.readthedocs.io