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_

從 README 開始，檢視 workerize-loader 相對於 greenlet 和 workerpool 的定位。釐清理想的使用情境，包括重疊的工作和 pooling，並說明 async 匯出的模組方法是否意味著存在限制 worker 數量的方式。完成的標準是 README 透過建議的比較回答這些問題。

由索引模型根據 Issue 內容生成。