Add installation instructions

These installation instructions obsolete the README generated by
backup/backup (which I'll modify in a later commit). These should be
fully working (barring test suite failures, which I'm already opening
bugs for), so if they don't, I'd like to hear about any problems.
Fix one of the instructions

Oops, this instruction wasn't quite right in its description.
Merge pull request #9 from Sophira/install-instructions

Add installation instructions