Improve INSTALL documentation

Additional clarification note added.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>

INSTALL

@@ -188,7 +188,7 @@ the exception of curl), so you can use either apt::
                     libghc-attoparsec-dev libghc-vector-dev \
                     libghc-snap-server-dev
 
-or ``cabal``::
+or ``cabal``, after installing a required non-Haskell dependency::
 
   $ apt-get install libpcre3-dev
   $ cabal install hslogger Crypto text hinotify regex-pcre \