Tidy up and convert documentation

- Add a proper markdown README file, partly based on
http://www.nerdoftheherd.com/tools/radiodld/help/contribute/
- Convert the release process from plain text to Markdown and update to
reflect the switch from svn to git.
- Rename the GPL license file to LICENCE.txt and move the short rtf
version of the license to the installer folder as this is the only place
it is used.