Add docs for cleanGit

docs/user-guide.md

@@ -119,7 +119,8 @@ unwanted builds (for instance when the contents of `dist-newstyle` are
 modified by cabal).  It is important that `src` is the root directory
 of the repo (so `cleanGit` can find the `.git` directory).  If the project
 is not at the root of the repo, then also pass `subDir` to `cleanGit` with the
-location of the project relative to `src` as a string. For example:
+location of the project relative to `src` as a string ([more information
+on using cleanGit](clean-git.md)). For example:
 
 ```
   src = pkgs.haskell-nix.haskellLib.cleanGit