doc/tex dir removed, but corresponding entries still exists

Update README, CHANGES, and Makefile.  Direct users to 
http://wiki.asterisk.org for documentation or to the 
AST.txt and AST.pdf included in the tarball.

(closes issue #18443)
Reported by: bas
Patches:
      changes.diff uploaded by lathama (license 1028)
      readme.diff uploaded by lathama (license 1028)
Tested by: lathama bas



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@305561 65c4cc65-6c06-0410-ace0-fbb531ad65f3