Document and release 0.5.0~beta-1 into experimental

One thing to note: The tarball you get from the artifacts in gitlab does not include the needed submodules (libgd) which is why I ran ninja -C _build dist to get a tarball which includes all the sources (this is also what gets uploaded to gnome's ftp btw).

For the next release upstream chatty (and calls as well btw) has to figure out how and where to provide the tarballs in a way that can be consumed by uscan