add pre-commit who use black formatting

GUIDE.md

@@ -58,3 +58,10 @@ class Mycheat(Hack):
 
 Please, use `black` before doing a pull request, as it's the formatter the
 project use.
+
+On Linux, you can copy the git hook stored in [githooks/](./githooks) who check
+the code, preventing any commit with bad formatting.
+
+```sh
+$ cp githooks/* .git/hooks/
+```

githooks/pre-commit

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+python3 -m black --check .