Add link to https://crab.fit/ in README

[hsandt]

I noticed that the README mentioned Self-hosting but, oddly enough, not the main instance of Crab Fit hosted at https://crab.fit/

Fortunately it's easy to find with a simple web search. It's also the repo name itself, but not so obvious since "fit" is not a common URL ending.

(posted after noticing the same issue in fork "Star Fit": drinkablebreeze#18)

EDIT: I forgot that you could put URL as metadata on a repo in the right side bar and that's exactly what this repo is already doing. Well, I suppose an extra link in README won't hurt.