Add use case

Signed-off-by: Richie Bendall <richiebendall@gmail.com>

index.d.ts

@@ -48,6 +48,8 @@ export type IterableOptions = BaseOptions & {
 	/**
 	Maximum number of promises returned by `mapper` that have resolved but not yet collected by the consumer of the async iterable. Calls to `mapper` will be limited so that there is never too much backpressure.
 
+	Useful whenever you are consuming the iterable slower than what the mapper function can produce concurrently. For example, to avoid making an overwhelming number of HTTP requests if you are saving each of the results to a database.
+
 	Default: `options.concurrency`
 	*/
 	readonly backpressure?: number;

readme.md

@@ -90,6 +90,8 @@ Minimum: `options.concurrency`
 
 Maximum number of promises returned by `mapper` that have resolved but not yet collected by the consumer of the async iterable. Calls to `mapper` will be limited so that there is never too much backpressure.
 
+Useful whenever you are consuming the iterable slower than what the mapper function can produce concurrently. For example, to avoid making an overwhelming number of HTTP requests if you are saving each of the results to a database.
+
 ##### stopOnError
 
 **Only for `pMap`**