Document process to pull in Prism updates

[amomchilov]

Originally posted by @st0012 in #42 (comment)

RBS makes use of some files from Prism (namely the defines.h and pm_constant_pool.c/h, and there's a growing desire to bring the two libraries closer together.

I wrote a scrappy sync_from_prism.rb script to pull in Prism's version of these files, and rename the identifiers (PRISM → RBS, pm_ → rbs_, etc.). It completely replaces the local copy of the file with the modified version from Prism. This undoes some local changes, which need to be discarded before committing the desired updates.

This script should be polished up, documented, and merged in under some folder like support/