enhance README to include ideal use case(s)

I came to `workerize-loader` for ease-of-use, but now I wonder about about pooling-type issues imposed by my present need for overlapping tasks.

So, I hope this is a helpful suggestion: _Clarify use case in_ `README`, _e.g._, `greenlet` < `workerize-loader` < `workerpool`.

For example, does the following feature imply a 'natural' way to limit the number of workers?

     _If exported module methods are already async, signature is unchanged_