Clarify top-level purpose of uvwasi 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.)