Documentation.md: add pre-commit hook sample (fsprojects#1392)

* Documentation.md: add pre-commit hook sample

Following https://stackoverflow.com/a/42345379/1623521

Fixes fsprojects#1207

* Add fair warning

Co-authored-by: nojaf <florian.verdonck@outlook.com>

docs/Documentation.md

@@ -1068,6 +1068,20 @@ Exclusion applies both to formatting and the format checking.
 
 See [CodeFormatter.fsi](../src/Fantomas/CodeFormatter.fsi) to view the public API of Fantomas.
 
+## A git pre-commit hook sample
+
+A very elegant and transparent way to use Fantomas is including it in a pre-commit git hook, by creating a `.git/hooks/pre-commit` file with:
+
+```
+#!/bin/sh
+git diff --cached --name-only --diff-filter=ACM -z | xargs -0 $HOME/.dotnet/tools/fantomas
+git diff --cached --name-only --diff-filter=ACM -z | xargs -0 git add
+```
+
+<small>This script assumes you have installed Fantomas globally as a [dotnet tool](https://www.nuget.org/packages/fantomas-tool/)</small>
+
+**Please use with caution** as [Fantomas is not without bugs](https://github.com/fsprojects/fantomas/issues?q=is%3Aissue+is%3Aopen+label%3A%22bug+%28soundness%29%22).
+
 ## FAKE Helpers
 
 Fantomas also exposes some less official helper functions when formatting code in FAKE scripts.