Add "yarl" to the "Migrating"-section in the README

I've recently had reason to curse out stdlib urllib.parse.url* and went looking for alternatives. I found `uri`, `furl` and `yarl`, the latter is missing from the "Migrating"-section in the README.

[YARL](https://pypi.org/project/yarl/) 

I really enjoy the snark in the migrating-section so here's hoping ya'll have the spoons to take a look :)