Update README.md with More Thorough Documentation

[julianworden]

I started working with this library because it was listed here, but the documentation was quite minimal and I ended up having to experiment a lot in order to figure out how to do most things. It's possible that not all of the info in this PR is 100% correct, but code examples such as these would've been very helpful for me when I first started using this library so I'm confident that at least some of this will be useful.

One thing I did remove was this section from the setup instructions:

defer {
    // it's important to shutdown the httpClient after all requests are done, even if one failed. See: https://github.com/swift-server/async-http-client
    try? httpClient.syncShutdown()
}

While I'm sure it'd be important to include this block, I cannot figure out how to actually use it in the context of an app. If I call it when the app is first initialized, I get an error message after sending my first message. Currently, I have a sample app running just fine without this block, so if someone would like to incorporate this block into my examples I'll leave that up to you.

Thanks!