Expand top-level wasmtime crate docs (#1955)

This is something I meant to do a long time ago but forgot to get around
to it! This commit updates the top-level documentation of the `wasmtime`
crate to have examples, more words, and generall be a bit more
up-to-date and complete.

crates/wasmtime/Cargo.toml

@@ -31,6 +31,7 @@ winapi = "0.3.7"
 
 [dev-dependencies]
 tempfile = "3.0"
+wasmtime-wasi = { path = "../wasi" }
 
 [badges]
 maintenance = { status = "actively-developed" }