Update organization of Documentation README

Change the flow of the Documentation README file to widen the focus of the
README.  Add links to other bits of documentation in the VistA repo

Add a table of contents directive to the top of the file.

OSEHRA-Id: http://issues.osehra.org/browse/OAT-149
Change-Id: I231553f75ccdcffb3b6bebe725cd72649a7ca352
Merge "Update organization of Documentation README"