Add IteratorStream docs

docs/iterator-stream.md

@@ -1,3 +1,16 @@
 # IteratorStream
 
-TBD
+`IteratorStream` allows converting an `Amp\Iterator` that yields strings into an `InputStream`.
+
+```php
+$emitter = new Emitter;
+
+asyncCoroutine(function () use ($emitter) {
+    for ($i = 0; $i < 10; $i++) {
+        yield new Delayed(1000);
+        yield $emitter->emit(".");
+    }
+});
+
+$inputStream = new IteratorStream($emitter->iterate());
+```