[lit-labs/virtualizer] Add documentation for the keyFunction property

[imlutr]

Hi! I was using the Lit virtualizer and noticed it was kind of slow. I dug in the Performance tab and I noticed that it basically uses repeat() for some of its work. Then I wondered about using keyFunction, to optimize the performance and sure enough, the library offers such functionality - and it seems to make a fair difference.

It'd be great if this was mentioned in the README for the virtualizer 😄.