Migrate the wasmtime shim README to /docs

[Mossaka]

the README from wasmtime shim contains a WASI HTTP demo application which I feel it's useful to put into our docs.

[Owais-04]

Is this still open? I'm new to open source contribution and would like to start with documentation. Thank you!

[Mossaka]

Hey @Owais-04 , yes this is still open! Thank you