Updates installation instructions

I've been taking a look into OWL and trying to run it.

INSTALL.md was a little outdated, so I am updating it as I go. Here is a PR with what I've got so far.

I am stuck in the tiler, though. By looking into `scripts/tiler.rb`, it seems to me that it needs some kind of input file with the changeset ids. How do I get/generate one of those?