Added contributing guidelines

Quite a few people are opening issues which have been resolved before, such as using multiple `<h1>` tags per page, and adding support for old versions of IE.

I suggest we add a [contributing guidelines file](https://github.com/blog/1184-contributing-guidelines) to this repositiory, and include a list of common issues that people might be tempted to bring up again.

When people go to open an issue or pull request, they will see this notice:

![](https://github-images.s3.amazonaws.com/skitch/issues-20120913-162539.jpg)

…which will link to the `CONTRIBUTING.md` file in the root of this repo.