add project updates (changes to data.json) as feed, fix #27

A simple solution to have an RSS feed on the page. This just shows the changes to the data.json file, as @jhuet suggested in #27. I think that’s better because simpler than having additional overhead via hooks or building a custom feed.
As you see the commit messages should be properly worded sentences, but that should be the case always.

Please review @libreprojects/contributors!