Document process_keywords handling of file table

[jtniehof]

I found this in the course of #102. The process_keywords column of file isn't well documented...it has something to do with populating the format when it isn't knowable from the metadata, but I'd think that was maybe more product_keywords. Need to dig into this and figure it out.

OS, Python version, and dependency version information:

Linux-4.15.0-163-generic-x86_64-with-Ubuntu-18.04-bionic
sys.version_info(major=2, minor=7, micro=17, releaselevel='final', serial=0)
sqlalchemy=1.1.11

Version of dbprocessing

#102 , i.e. 1d5a938 or jtniehof/dbprocessing:document_tables

Closure condition

This issue should be closed when appropriate documentation describing this functionality (not just the column) is merged.

[balarsen]

On this one there were two ways to specify keywords to processes, as with other places I think likely we just need one and a decision could be made to remove one of the two ways.