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.
    
    Fixes #43.
    
    Signed-off-by: Justin W. Flory (he/him) [Fedora Project] <jwf@fedoraproject.org>
    
    Change IRC badge to a Matrix chat room badge
    
    Signed-off-by: Justin W. Flory (he/him) [Fedora Project] <jwf@fedoraproject.org>