Include plugin README with docs

Author: sellout

plugin/categorifier-plugin.cabal

@@ -5,6 +5,7 @@ version:        0.1
 description:    GHC plugin for Compiling to Categories
 homepage:       https://github.com/con-kitty/categorifier#readme
 bug-reports:    https://github.com/con-kitty/categorifier/issues
+extra-doc-files: README.md
 build-type:     Simple
 tested-with:    GHC==8.10.1, GHC==8.10.7, GHC==9.0.1, GHC==9.2.1, GHC==9.2.2, GHC==9.2.8