Write a README for cargo-shuttle

[mikeando]

In the [Deploying](https://docs.rs/shuttle-service/latest/shuttle_service/#deploying) section there is a link to cargo shuttle
which links to https://docs.rs/crate/cargo-shuttle/latest.

That leads to a valid page, but its main content is cargo-shuttle-0.2.2 is not a library. which is not particularly welcoming.

[brokad]

I agree. We should link to GH with a README.md for the cargo-shuttle crate instead.

[nahuakang]

@brokad @christoshadjiaslanis Looking for something to do :) If this is still required, can you let me know what's expected on the README (like a short bullet-point outline) and I can draft something?

[chesedo]

@nahuakang for anyone installing the cli tool this might still be nice though. A short bullet-point outline with a short description for each sub-command will be great.

Since the cli is not stable yet, I fear a proper documentation will create extra maintenance in the near future.

[nahuakang]

@chesedo Thanks :) Then we'll keep it short and simple until the CLI is stable?

[chesedo]

@nahuakang Yes, please 😄