Converted `how-to-contribute` to CONTRIBUTING.md

GitHub supports `CONTRIBUTING.md` files that show up at the creating new issue/PR pages, like this:

![Contributing guidelines link](https://cloud.githubusercontent.com/assets/177485/3954958/9c20e914-26fc-11e4-9d61-299baf0eaec6.png)

This PR converts the existing `how-to-contribute` page to such file, and then moved it back using a relative symlink, so it would still be there as a proper page on the site. It also removes the metadata from this page, so it won't confuse users when they would read the contributing guidelines.

This is made by adding a handler for pages without title sets (they gain the title from their file names, like in Jekyll) and adding a hardcoded `order` for how-to-contribute page. Maybe there is a bette way to do the latter, but hey.