Add README.md to extra-doc-files

Author: harendra-kumar

packdiff.cabal

@@ -15,7 +15,7 @@ maintainer:          streamly@composewell.com
 copyright:           2022 Composewell Technologies
 -- category:
 build-type:          Simple
-extra-source-files:  CHANGELOG.md
+extra-doc-files:  README.md, CHANGELOG.md
 
 common compile-options
     default-language: Haskell2010