Add prepare-commit.sh setup step in how to contribute patches

Also use real step numbers so you don't need a markdown renderer
to read the content

CONTRIBUTING.md

@@ -15,9 +15,10 @@ You can help us **by submitting bug reports or fixing bugs** in the [QGIS bug tr
 If you wish to contribute patches you can:
 
 1. [fork the project](https://help.github.com/forking/)
-1. make your changes
-1. commit to your repository
-1. and then [create a pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/).
+2. Install the pre-commit hook: `ln -s ../../scripts/prepare_commit.sh .git/hooks/pre-commit`
+3. make your changes
+4. commit to your repository
+5. and then [create a pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/).
 
  The development team can then review your contribution and commit it upstream as appropriate.