Add Link To Web Documentatio Of distcc-3.1

Add a link to the latest released (or rather tagged) documentation.  The
rawgit.com service requests that we "Use a specific tag or commit hash
in the URL (not a branch)" for production code.  For development we are
permitted to use other things I guess.

The old website is gone as is the google code backup of it.  This change
would make the old documentation more visible as long as GitHub and
RawGit are online.

README.md

@@ -2,7 +2,8 @@
 
 by Martin Pool
 
-http://distcc.org/
+Current Docuemnts: https://cdn.rawgit.com/distcc/distcc/9a09372bd3f420cdd7021e52eda14fa536a3c10e/doc/web/index.html
+Formally http://distcc.org/
 
 "pump" functionality added by
 Fergus Henderson, Nils Klarlund, Manos Renieris, and Craig Silverstein (Google Inc.)