Improve documentation

[rs028]

• Add LaTeX files (including figures) of the manual to the doc/ folder, so that it can be updated.
• Add a CHANGELOG.md file to the main directory summarizing the changes since version 7.4.1
• Add a CREDITS.md file to the main directory with the names of everybody who has contributed code over the years (this may require some digging, so let's start with the current list of names).
• Any outstanding issues and things to do/check/fix/improve (and other things that need to be kept in mind for future development) should be listed as open issue on the github issues page.
• A couple of items are listed in src/changelog.txt and should be moved to the github issues page.
• There needs to be a little more explanation on how to use the model than the few lines in the README file.
• Expand text in the README and convert to markdown (README.md ).
• The scripts in plots/ require ferret, which should be listed as requirement in the readme file.
• Document usage of namelists/ and param/ (see issue namelists and param #21).