feat: Implement concurrency (#38)

* feat: Add concurrency limit

* Refactor to clean up logic

* Fix type error

* Remove @types/debug

README.md

@@ -103,6 +103,41 @@ const text = await retrier.retry(() => fs.readFile("README.md", "utf8"));
 
 The `retry()` method will either pass through the result on success or wait and retry on failure. Any error that isn't caught by the retrier is automatically rejected so the end result is a transparent passing through of both success and failure.
 
+### Setting a Timeout
+
+You can control how long a task will attempt to retry before giving up by passing the `timeout` option to the `Retrier` constructor. By default, the timeout is one minute.
+
+```js
+import fs from "fs/promises";
+
+const retrier = new Retrier(error => {
+    return error.code === "ENFILE" || error.code === "EMFILE";
+}, { timeout: 100_000 });
+
+const text = await retrier.retry(() => fs.readFile("README.md", "utf8"));
+```
+
+When a call times out, it rejects the first error that was received from calling the function.
+
+### Setting a Concurrency Limit
+
+When processing a large number of function calls, you can limit the number of concurrent function calls by passing the `concurrency` option to the `Retrier` constructor. By default, `concurrency` is 1000.
+
+```js
+import fs from "fs/promises";
+
+const retrier = new Retrier(error => {
+    return error.code === "ENFILE" || error.code === "EMFILE";
+}, { concurrency: 100 });
+
+const filenames = getFilenames();
+const contents = await Promise.all(
+    filenames.map(filename => retrier.retry(() => fs.readFile(filename, "utf8"))
+);
+```
+
+### Aborting with `AbortSignal`
+
 You can also pass an `AbortSignal` to cancel a retry:
 
 ```js