Add optional plugins document. (#1134)

docs/README.md

@@ -5,6 +5,7 @@
     * [Quick start](en/Quick-start.md)
     * [Supported middlewares, frameworks and libraries](Supported-list.md)
       * [How to disable plugins?](en/How-to-disable-plugin.md)
+      * [Optional plugins](en/Optional-plugins.md)
   * Advanced Features
     * [Override settings through System.properties or System.env](en/Setting-override.md)
     * [Direct uplink and disable naming discovery](en/Direct-uplink.md)

docs/en/Optional-plugins.md

+# Optional plugins
+Optional plugins could be provided by source codes or in `optional-plugins` folder under agent.
+
+For using these plugins, you need to compile source codes by yourself, or copy the certain plugins to `/plugins`.
+
+## Spring bean plugin
+This plugin allows to trace all methods of beans in Spring context, which are annotated with
+`@Bean`, `@Service`, `@Component` and `@Repository`.
+
+- Why does this plugin optional?
+Tracing all methods in Spring context all creates a lot of spans, which also spend more CPU, memory and network.
+Of course you want to have spans as many as possible, but please make sure your system payload can support these.
+
+## Oracle and Resin plugins
+These plugins can't be provided in Apache release because of Oracle and Resin Licenses.
+If you want to know details, please read [Apache license legal document](https://www.apache.org/legal/resolved.html)
+
+- How should we build these optional plugins in local?
+
+1. Resin 3: Download Resin 3.0.9 and place the jar at `/ci-dependencies/resin-3.0.9.jar`.
+1. Resin 4: Download Resin 4.0.41 and place the jar at `/ci-dependencies/resin-4.0.41.jar`.
+1. Oracle: Download Oracle OJDBC-14 Driver 10.2.0.4.0 and place the jar at `/ci-dependencies/ojdbc14-10.2.0.4.0.jar`.
+
+