small tweaks to overview doc

doc/README

-This directory contains some high level documentation for the code.  We should strive to keep it up-to-date, but don't take it as the absolute truth.
+This directory contains some high level documentation for the code.
 
-A good place to start is 'overview.md'
+WARNING: much of this is out-of-date. It stil may be helpful, though.

doc/overview.md

-# Documentation for edX code (mitx repo)
+# Documentation for edX code (edx-platform repo)
 
 This document explains the general structure of the edX platform, and defines some of the acronyms and terms you'll see flying around in the code.