Add contribution documentation

[TysonAndre]

1. ./main.sh is the recommended way to run webgen (Dockerized). Everything except src/downloads.page should be fine
2. There is a gh-pages branch (http://uniqush.github.io/uniqush-www/) to track changes to snapshots of generated HTML at given points in time (E.g. to make sure that the sidebar doesn't accidentally change or parts of the site aren't missing content). Not sure if that's the approach I want to use.
   It's possible to check out the branch and make a PR against it to update it with the new contents of out/, excluding rpms
3. Link to the docs for the old webgen version this uses.