Add CHANGES.md and README.md to sdist tarball

Author: liskin

X11-xft.cabal

@@ -13,6 +13,8 @@ description:   A Haskell bindings to the X Font library. With it, Haskell X11
                also provide minimal bindings to Xrender parts.
 build-type:    Simple
 tested-with:   GHC == 7.10.3 || == 8.0.2 || == 8.2.2 || == 8.4.4 || == 8.6.5 || == 8.8.4 || == 8.10.4 || == 9.0.1 || == 9.2.1
+extra-source-files: CHANGES.md
+                    README.md
 
 flag pedantic
   description: Be pedantic (-Werror and the like)