Update installation instructions for 1.2.1
Update installation instructions for 1.2.1

This also ties up any loose ends, including where to find the tarball
and how to contribute.

tejasmanohar/rcm,TheLinuxKitten/rcm,devm33/rcm,fgandellini/rcm,ali/rcm,Frost/rcm,zachlatta/rcm,tejasmanohar/rcm,fgandellini/rcm,TheLinuxKitten/rcm,ali/rcm,devm33/rcm,ali/rcm,zachlatta/rcm,thoughtbot/rcm,Frost/rcm,Frost/rcm,fgandellini/rcm,devm33/rcm,Frost/rcm,thoughtbot/rcm,ali/rcm,TheLinuxKitten/rcm,thoughtbot/rcm,zachlatta/rcm,tejasmanohar/rcm,zachlatta/rcm,tejasmanohar/rcm,fgandellini/rcm,TheLinuxKitten/rcm,devm33/rcm