doc: add WebGL section on README.md

Author: netpyoung

README.md

@@ -61,6 +61,12 @@ ex)
 
 ![animation.webp](./animation.webp)
 
+## WebGL
+
+- WebGL's [System.Threading.Tasks](https://docs.microsoft.com/dotnet/api/system.threading.tasks.task?view=net-6.0) async based logic is not stable. If you want to use async based logic for WebGL build, try to use [Cysharp/UniTask](https://github.com/Cysharp/UniTask).
+  - ref: <https://forum.unity.com/threads/async-await-and-webgl-builds.472994/>
+- For WebGL build, this repo copied [webmproject/libwebp](https://github.com/webmproject/libwebp) directly. It will be more cleanable if it can be support prebuilt. But It needs more investigate.
+
 ## LICENCE
 
 ### webp