Add link to git-hook tips in PR template After a conversation with @aneeshusa on IRC starting [here-ish](http://logs.glob.uno/?c=mozilla%23servo#c430067), info about a [git hook](https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks) that runs `test-tidy` on commit (or push) was added to the wiki [here](https://github.com/servo/servo/wiki/Github-workflow#test-tidy-commit-hook). A suggestion was made to also add a link to this in the PR template. Source-Repo: https://github.com/servo/servo Source-Revision: d53507f747f7122dc520f5e4a374ee1ad955aa5d