README: Update deprecated URLs, update IRC -> Matrix (fixes #43)
This is a periodic house-keeping sweep of our README. I updated our chat
room to point to our Matrix room since it has our pretty logo. It also
makes sure people can find the right links to our stuff. Better to fix
this kind of thing sooner than later.
Signed-off-by: Justin W. Flory (he/him) [Fedora Project] <email@example.com>