Document feature comparison with `envy`

[ypoluektovich]

After seeing envious in TWIR's "crate of the week" segment, I immediately thought of another, very similarly named library that does "the same thing" and that I've been using for my pet projects for a while now: https://github.com/softprops/envy. And I imagine I'm not the only one. Can we have a doc section (maybe not even in README, a separate file is fine) with a feature comparison?