add README for compatible module

[diecui1202]

What is the purpose of the change

add README

Brief changelog

dubbo-compatible/README.md

Verifying this change

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GITHUB_issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a GITHUB issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [Dubbo-XXX] Fix UnknownException when host config not exist #XXX. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add integration-test in test module.
• Run mvn clean install -DskipTests & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
• If this contribution is large, please follow the Software Donation Guide.

[codecov-io]

Codecov Report

Merging #2019 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #2019      +/-   ##
===========================================
- Coverage     52.14%   52.1%   -0.04%     
+ Complexity     4902    4900       -2     
===========================================
  Files           562     562              
  Lines         25216   25216              
  Branches       4449    4449              
===========================================
- Hits          13150   13140      -10     
- Misses        10060   10068       +8     
- Partials       2006    2008       +2

Impacted Files	Coverage Δ	Complexity Δ
...he/dubbo/remoting/transport/netty/NettyClient.java	72.88% <0%> (-10.17%)	12% <0%> (-1%)
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	41.66% <0%> (-4.17%)	3% <0%> (ø)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	8% <0%> (-1%)
...he/dubbo/registry/multicast/MulticastRegistry.java	56.06% <0%> (-1.68%)	38% <0%> (-3%)
...bo/remoting/transport/netty/NettyCodecAdapter.java	53.12% <0%> (-1.57%)	3% <0%> (ø)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	55.69% <0%> (+3.79%)	13% <0%> (+2%)	⬆️
...org/apache/dubbo/rpc/filter/ActiveLimitFilter.java	88.88% <0%> (+5.55%)	7% <0%> (+1%)	⬆️

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