Create git pre-commit hook for preventing tabs

In order to prevent use of tabs, I created a pre-commit hook, aborting commit if tabs are in the commit.

+ Update documentation in CONTRIBUTING.md to include git hooks

Should we provide a script (or a Makefile) which creates links to the hooks or should we let the users do the links them-selves?
Should the documentation of the hooks be in CONTRIBUTING.md or elsewhere ? README.md ? Just asking.