initial version
Allow setting github repo url in _config.yml
Add a minimal TODO
Minor correction and tweak to _README
Anoter TODO
Do not link to PDF manual by default

PDFs are bad for storing in a git repository, and most people
won't be interested in downloading this from us anyway. So do
not force package authors to put it onto their gh-pages (they
still can do so by modifying the template if they want; OTOH,
users can download the PDF manual from gap-system.org...)
Setup gh-pages based on GitHubPagesForGAP