Add security.md

[ralf0131]

Add security.md

[codecov-io]

Codecov Report

Merging #4165 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4165      +/-   ##
============================================
+ Coverage     62.88%   62.91%   +0.03%     
  Complexity      505      505              
============================================
  Files           768      768              
  Lines         33064    33064              
  Branches       5220     5220              
============================================
+ Hits          20792    20802      +10     
+ Misses         9869     9863       -6     
+ Partials       2403     2399       -4

Impacted Files	Coverage Δ	Complexity Δ
...mmon/threadpool/support/AbortPolicyWithReport.java	83.78% <0%> (-5.41%)	0% <0%> (ø)
...dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java	80.14% <0%> (-0.74%)	0% <0%> (ø)
...pache/dubbo/remoting/transport/AbstractServer.java	48.95% <0%> (+1.04%)	0% <0%> (ø)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	81.6% <0%> (+1.53%)	0% <0%> (ø)	⬇️
...ng/exchange/support/header/HeartbeatTimerTask.java	78.94% <0%> (+5.26%)	0% <0%> (ø)	⬇️
...apache/dubbo/common/config/ConfigurationUtils.java	72% <0%> (+8%)	0% <0%> (ø)	⬇️
.../apache/dubbo/remoting/transport/AbstractPeer.java	69.56% <0%> (+10.86%)	0% <0%> (ø)	⬇️

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[carryxyh]

LGTM.