fix export, add runtime balancer for async tasks

doc/async.md

@@ -0,0 +1,39 @@
+## Asynchronous Runtime Balancer
+
+> The async processing model is automatically observed by a runtime balancer to prevent any blocking issues, even on page load.
+
+The most methods of each index type provides an async version, e.g.:
+
+- addAsync()
+- updateAsync()
+- removeAsync()
+- searchAsync()
+
+All those async versions always return a `Promise`, although a callback can be passed additionally as the <u>last parameter</u>.
+
+When calling async methods of the index, a runtime balancer observe the current event loop and will pass to the next event loop automatically.
+
+You can control how early the process should move over to the next event loop by passing the option property `priority`:
+
+```js
+const index = new Index({
+    // a value between 1 and 9
+    priority: 4
+});
+```
+
+The lowest valid priority number is `1` and is typically known as `idle` (event loop cycles by native ~4ms). The default priority is `4` which is optimized for non-blocking user interfaces within a browser (event loop cycles every ~40ms).
+When you have some very smooth running animation you should use a priority of `2` to keep the animation running by 60 fps without any stutter. Targeting 120 fps or higher you should use `1`.
+
+On Node.js you can slightly increase this priority e.g. to `6`, because here there is no UI involved. 
+A priority value of `9` will cycle the event loop on every ~500ms which is the maximum recommended blocking time. You should not use a value higher than this.
+
+Do not forget to `await` on <u>every</u> async task you apply to the index:
+
+```js
+for(let i = 0; i < 99999999; i++){
+    await index.addAsync(i, "test " + i);
+}
+```
+
+You can perform queries to the index during any other async batch is running.