Clarify top-level purpose of libuv in README.md
Clarify top-level purpose of libuv in README.md

This adds a diagram provided by @tniessen to the README, which I
believe is very helpful (at least it would have saved me quite some
time, puzzling over why uvwasi wasn't what I hoped it would be.)

(I believe it's also good to clarify that uvwasi is not useful to
people with libuv-based code who wish to run that code in WASI
containers. I'm sure many in that situation would type in "libuv" and
"wasi" and get here as the top hit.)

nodejs/uvwasi