Update the top-level README.md and embedding documentation. (#508)

* Update the top-level README.md and embedding documentation.

wasmtime-api is now the primary external API crate, so recommend that
instead of wasmtime-jit.

Also, enable wasmtime-api's C API by default, so that it shows up on
docs.rs, and to make it easier to use.

And, add basic embedding documentation and link to it from the
README.md. Credit to @yurydelendik for the content.

* Use the new wasm-c-api URL.

* Don't pass --features wasm-c-api, as it is now on by default.

wasmtime-api/src/lib.rs

@@ -14,7 +14,6 @@ mod trap;
 mod types;
 mod values;
 
-#[cfg(feature = "wasm-c-api")]
 pub mod wasm;
 
 #[macro_use]