Add documentation for installing Cache.dat file

Add a rST documentation file that instructs the user on how to use a
downloaded CACHE.dat file for VistA instead of importing from the OSEHRA
Repository.  Add a line in the README file to ensure that the file is
known.

OSEHRA-Id: http://issues.osehra.org/browse/OAT-114
Change-Id: I6488ee716990ff1cc171e62fab007396cd41bf5a