add README for compatible module (apache#2019)

dubbo-compatible/README.md

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+### dubbo-compatible
+
+Hi, all
+
+From 2.7.x, `Dubbo` has renamed package to `org.apache.dubbo`, so `dubbo-compatible` module is provided.
+
+For compatibility with older versions, we provider the following most popular APIs(classes/interfaces):
+
+* com.alibaba.dubbo.rpc.Filter / Invocation / Invoker / Result / RpcContext / RpcException
+* com.alibaba.dubbo.config.annotation.Reference / Service
+* com.alibaba.dubbo.config.spring.context.annotation.EnableDubbo
+* com.alibaba.dubbo.common.Constants / URL
+* com.alibaba.dubbo.common.extension.ExtensionFactory
+* com.alibaba.dubbo.common.serialize.Serialization / ObjectInput / ObjectOutput
+* com.alibaba.dubbo.cache.CacheFactory / Cache
+* com.alibaba.dubbo.rpc.service.EchoService / GenericService
+
+The above APIs work fine with some unit tests in the test root. 
+
+Except these APIs, others provided in `dubbo-compatible` are just bridge APIs without any unit tests, they may work with wrong. If you have any demand for them, you could: 
+
+* Implement your own extensions with new APIs. (RECOMMENDED) 
+* Follow `com.alibaba.dubbo.rpc.Filter` to implement bridge APIs, and then contribute to community. 
+* Open issue on github.
+
+By the way, We will remove this module some day, so it's recommended that implementing your extensions with new APIs at the right time. 
+
+Now we need your help: Any other popular APIs are missing?
+
+For compatible module, any suggestions are welcome. Thanks.