Add a `CONTRIBUTING.md` file

Extract the information on contributing from the README to a dedicated
new root file CONTRIBUTING. This is both ensuring the README remains
concise and also allows to take advantage of GitHub's usability feature
that prompts people to read a repo's guidelines before reporting issues
or opening pull requests.

----
@necolas - I've taken the liberty to semantically apply the changes of your respective commit [Add a `CONTRIBUTING.md` file](https://github.com/h5bp/html5-boilerplate/commit/2c079ae144cbab66d21edc99dded693a910d9fc2#CONTRIBUTING.md) here as well to encourage/spread the use of this smart and helpful GitHub usability feature:
> Move the information on contributing from the bundled documentation to a
new root file - CONTRIBUTING. This is both a more meaningful location
(not part of code documentation) and allows us to take advantage of
GitHub's latest UI changes that prompt people to read a repo's
guidelines before reporting issues or opening pull requests.

This suggested/required minor changes to the header naming/structure to allow for cross linking from the now more concise README.

In particular, I've unified all headers to verb rather than noun style, which is definitely a matter of preference (I personally prefer nouns actually, but usability seems to suggest direct engagement to be more appropriate).

Likewise I've dropped the headings _Rules of Engagement_ and _Do not..._ in favor of _Contributing to this project_ and _Using the issue tracker_ to both better match the respective GitHub UI for [Contributing Guidelines](https://github.com/blog/1184-contributing-guidelines) and also be a bit less formal/commanding at first sight (the actual bullet points remain the same though), which hopefully encourages readers to ingest the respective explanations more open and thoroughly.