Add Lefthook to the Gemfile and improve documentation
What does this MR do?
I think Lefthook has proven to work well, and I don't see a good reason not to add it to the development dependencies (similarly to how we install it in the GDK: gitlab-development-kit!1500 (diffs)).
I've also improved the documentation by describing the current configuration.
Edited by Rémy Coutable