update README

Made 3 distinct steps, clarified packaging, moved running bot at end (not needed by most people), ensured correct capitations.

Diff is confusing; easier just to [read directly](https://github.com/tomByrer/libgrabber/blob/master/README.md)
"that" = "libgrabber".  Might be better as 2 sentences?

> libgrabber is a bot that runs on jsDelivr's servers.  It is responsible to auto-update projects that we host.