Add AsyncParsableCommand to README

[robertmryan]

Update to README and markdown documentation to include easily discovered references to AsyncParsableCommand.

In issue #561, I noted that AsyncParsableCommand was not easily discovered. This pull request attempts to remedy this by adding simple references to it in README.md, ArgumentParser.md, and Validation.md. In GettingStarted.md I went so far as to add a new section with an example.

Please note, this only includes documentation changes and does not include any code changes. Therefore, there are no tests for this PR.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test

[rismay]

This is really hard to discover without documentation. Please merge if possible.

[robertmryan]

FWIW, the status checks said it was out of date, so I went back to my fork and updated it (while preserving my changes). I hope this helps…

[natecook1000]

@swift-ci Please test