Recommend saying something about pandoc-crossref versus pandoc-xnos in readme

[robert-dodier]

The readme for this project mentions that it is inspired by pandoc-xnos. Since pandoc-xnos appears to cover the same ground, I think it would help users understand the advantages of pandoc-crossref if there were a few words explaining in what way pandoc-crossref differs from pandoc-xnos.

[lierdakil]

If you want to make a detailed comparison, feel free, I'll be happy to add a link to it in the readme. At the time this project was created, there wasn't pandoc-xnos in the first place, only two separate rather rudimentary filters for figures and equations (not tables IIRC, but don't quote me on that, it was almost 10 years ago, the details are extremely fuzzy). And I haven't really looked at the alternatives since I started working on this.