enhance README to include ideal use case(s) #36

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