Add notes about fourmolu formatter

README.md

@@ -158,3 +158,16 @@ All other files in ["docs" directory](docs/) require manual corrections.
     - bash: `source completions/git-elegant.bash`
     - zsh: `source completions/_git-elegant`
 2. run `git-elegant <some>` and press Tab twice
+
+### Haskell code formatting
+We use fourmolu as a formatting tool for haskell code.
+Fourmolu works in pipeline fashion.
+To reformat source code use:
+```shell
+$ git ls-files -z '*.hs' | xargs -0 fourmolu --mode inplace
+```
+
+To check if everything is formatted correctly, use:
+```shell
+$ fourmolu --mode check app src test
+```