Add plugin test framework doc (#3786)

* Add SkyWalking plugin test framework doc

* Remove Chinese.

* Add a description to the env variable, CASE_SERVER_IMAGE_VERSION

* Fix typo

* Change paragraph structure.

docs/README.md

@@ -76,6 +76,11 @@ If you have been familiar with SkyWalking, you could use this catalog to find th
       * [Dynamic Configuration](en/setup/backend/dynamic-config.md). Make configuration of OAP changed dynamic, from remote service or 3rd party configuration management system.
       * [Uninstrumented Gateways](en/setup/backend/uninstrumented-gateways.md). Configure gateways/proxies that are not supported by SkyWalking agent plugins, to reflect the delegation in topology graph.
     * [UI setup document](en/setup/backend/ui-setup.md).
+* [Contributing Guides](en/guides/README.md). Guides are for PMC member, committer or new contributor. At here, you can know how to contribute from beginning.
+  * [Contact us](en/guides/README.md#contact-us). Guide users about how to contact the official committer team or communicate with the project community.
+  * [Process to become official Apache SkyWalking Committer](en/guides/asf/committer.md). How to become an official committer or PMC member.
+  * [Compiling Guide](en/guides/How-to-build.md). Follow this to compile the whole project from the source code.
+  * [Agent plugin development guide](en/guides/Java-Plugin-Development-Guide.md). Guide the developer to write their plugin, and follow [plugin test requirement](en/guides/Plugin-test.md) when you push the plugin to the upstream.
 * [Protocols](en/protocols/README.md). Protocols show the communication ways between agents/probes and backend. Anyone, interested in uplink telemetry data, definitely should read this.
 * [FAQs](en/FAQ/README.md). Include a manifest, including already known setup problems, secondary developments experiments. When  you are facing a problem, check here first.
 

docs/en/guides/Java-Plugin-Development-Guide.md

@@ -365,7 +365,7 @@ Please follow there steps:
 1. Create sub modules under `apm-sniffer/apm-sdk-plugin` or `apm-sniffer/optional-plugins`, and the name should include supported library name and versions
 1. Follow this guide to develop. Make sure comments and test cases are provided.
 1. Develop and test.
-1. Provide the automatic test cases. Read existing cases in `test/plugin/scenarios`, and add your new one.
+1. Provide the automatic test cases. Learn `how to write the plugin test case` from this [doc](Plugin-test.md)
 1. Send the pull request and ask for review. 
 1. The plugin committers approve your plugins, plugin CI-with-IT, e2e and plugin tests passed.
 1. The plugin accepted by SkyWalking.