[pixi] Added note on status of runtime.

spine-ts/spine-pixi/README.md

@@ -1,3 +1,5 @@
+> **Note**: this runtime is a work in progress. Please follow this issue to get alerted once the runtime is deemed complete. https://github.com/EsotericSoftware/spine-runtimes/issues/2305
+
 # spine-ts Pixi.js
 
-Please see the top-level [README.md](../README.md) for more information.
+Please see the top-level [README.md](../README.md) for more information.