linting: Document how to use planemo lint in a git pre-commit hook

Once planemo has a useful lint command, we should all start using it with every commit automatically via a pre-commit hook.

Depending on the lint speed, this may require some cleverness to only test tools/files which have changed since the last commit - much like how pep8 checking is often restricted to only the Python files with changes, e.g. https://github.com/cbrueffer/pep8-git-hook