Split contributing-related information into new top-level CONTRIBUTING file

`README.md` contains a lot of useful information, but it is too long and most people will never scroll all the way down.
*Proposal:* let's split it so that all information concerning developers or bug reports can now be found in `CONTRIBUTING.md`.

 GitHub [recognizes this file](https://github.com/blog/1184-contributing-guidelines) and displays a prominent, yellow-highlighted link to it for everyone who is trying to report a bug or submit a pull-request, making it much for likely that contributors will find this information. This should help cut down on duplicate and incomplete bug reports:

![screenshot of CONTRIBUTING.md](https://camo.githubusercontent.com/71995d6b0e620a9ef1ded00a04498241c69dd1bf/68747470733a2f2f6769746875622d696d616765732e73332e616d617a6f6e6177732e636f6d2f736b697463682f6973737565732d32303132303931332d3136323533392e6a7067)

 Of course, `README.md` now contains a link to the new file in the place where this information was previously found (GitHub now [supports relative links](https://github.com/blog/1395-relative-links-in-markup-files) in markdown).