Added INSTALL instructions about older cabal versions.

IT may be necessary to update cabal, since older versions
don't take into account the (new) possibility of changed
metadata on Hackage.

INSTALL

@@ -24,6 +24,11 @@ Quick install
 
     This procedure will install the released version of pandoc,
     which will be downloaded automatically from HackageDB.
+    (Note:  if this step fails, and you are using an older version
+    of the Haskell Platform, e.g. on Debian stable, you may need to
+    upgrade your version of cabal: `cabal install cabal-install`,
+    then try `cabal update && cabal install pandoc` again.)
+
     If you want to install a modified or development version
     of pandoc instead, switch to the source directory and do
     as above, but without the 'pandoc':