Add a short top-level README

README

+This Cabal darcs repository contains multiple packages:
+
+ * cabal/          -- the Cabal library package
+ * cabal-install/  -- the cabal-install package containing the 'cabal' tool.
+
+See the README in each subdir for more details.
+
+The canonical upstream repo lives at http://darcs.haskell.org/cabal/