add the notice of checkstyle

[lexburner]

What is the purpose of the change

code check style needs more details to avoid to confuse the contributor.

Brief changelog

CONTRIBUTING.md

[lovepoem]

Looks like dubbo/CONTRIBUTING.md and http://dubbo.apache.org/en-us/docs/developers/guide_dev.html are duplicates. Can we just maintain one?

[lovepoem]

dubbo/FAQ.md and http://dubbo.apache.org/en-us/docs/developers/user-guide/faq_dev.html are also duplicates

[codecov-io]

Codecov Report

Merging #2983 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2983      +/-   ##
==========================================
+ Coverage   63.95%   63.98%   +0.03%     
==========================================
  Files         584      584              
  Lines       26059    26059              
  Branches     4562     4562              
==========================================
+ Hits        16666    16675       +9     
+ Misses       7217     7204      -13     
- Partials     2176     2180       +4

Impacted Files	Coverage Δ
...ache/dubbo/remoting/transport/AbstractChannel.java	75% <0%> (-12.5%)	⬇️
.../remoting/transport/netty4/NettyClientHandler.java	75% <0%> (-11.12%)	⬇️
...in/java/org/apache/dubbo/common/utils/JVMUtil.java	73.58% <0%> (-7.55%)	⬇️
.../apache/dubbo/remoting/transport/AbstractPeer.java	58.69% <0%> (-6.53%)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	80.07% <0%> (-1.13%)	⬇️
...pache/dubbo/remoting/transport/AbstractServer.java	47.91% <0%> (-1.05%)	⬇️
...rg/apache/dubbo/common/timer/HashedWheelTimer.java	61.67% <0%> (-0.7%)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	68.45% <0%> (+0.67%)	⬆️
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	66.66% <0%> (+0.83%)	⬆️
...dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java	79.41% <0%> (+2.2%)	⬆️
... and 5 more

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[lovepoem]

Look good to me

[lexburner]

We should maintain two places, the master trunk must own the Contributing.md, i will copy this change to the dubbo blog later.