README: Use detailed markdown README

Previously we had a sparse README that just pointed to the
wiki. While wikis are great, the README is where effort
should be put for a first impression.

Added some useful links, tutorials, and communication tips
for potential contributors.
1 job for master in 14 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #233480
flatpak-documents

00:14:33