Restructuring documentation

Finally got around to restructuring the documentation in this repository, which was getting a bit long and unwieldy. No code changes here, this is all markdown only.

* Split off the documentation for each script into a `README.md` file of it's own, kept in that script's subdirectory.
* Consolidated the example plots from each script into thumbnails in the root readme, for a fast overview.
* Added SciLifeLab credits to the bottom of each readme file.
* Write a `CONTRIBUTING.md` readme file, describing how stuff should be added, to keep consistency.