Update the README.md.

Author: armiol

README.md

@@ -1,6 +1,18 @@
 # Spine Web
 
+[![Ubuntu build][ubuntu-build-badge]][gh-actions]
+
+[gh-actions]: https://github.com/SpineEventEngine/web/actions
+[ubuntu-build-badge]: https://github.com/SpineEventEngine/web/actions/workflows/build-on-ubuntu.yml/badge.svg
+
 The Spine library is intended for interactions between a web server and a JavaScript client.
 
 This repository contains both Spine web [API](./web/README.md) and its 
-[implementation](./firebase-web/README.md) based on Firebase Realtime Database. 
+[implementation](./firebase-web/README.md) based on Firebase Realtime Database.
+
+### Java Version
+
+Starting version `2.0.0-SNAPSHOT.69`, the server-side modules of this library are built with Java 11
+compilation target. Therefore, the consumer applications have to use Java 11 or higher.
+
+Prior versions were build with Java 8.