Resolve #152

Improve the documentation of OFTest. This involves a large number of changes:

* Converting of 'README' to Markdown.
* Merging of 'DESIGN' and 'STYLE', and subsequent conversion to Markdown to produce 'CONTRIBUTING.md'.
* General restructuring of both docs (i.e. developer-orientated content -> `CONTRIBUTING.md`, user-orientated to `README.md`)
* Addition of new content to both docs. This content is part-original, part-based on older web resources (see commit messages).
* Renaming of 'doc' to 'docs', per Python conventions.
* Other minor changes