Add @packageDocumentation to readme

The newer repos with auto-generated docs tend to add a `@packageDocumentation` block to the index.ts file with a brief overview of the module.

It would be great to automatically add the rendered version of this to the README.md file with a link to the the api docs. This way we keep it in sync automatically, it's easier for users to find and we can apply existing linting tools to it as well.