Add security policy

[nobodyiam]

It's better to have a clear and official way for people to report security issues.

Fixes https://github.com/sofastack/sofa-rpc/issues/1290

[codecov[bot]]

Codecov Report

Merging #1293 (64c85df) into master (9eff2f6) will increase coverage by 0.00%. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1293   +/-   ##
=========================================
  Coverage     72.00%   72.00%           
  Complexity      784      784           
=========================================
  Files           415      415           
  Lines         17651    17651           
  Branches       2753     2753           
=========================================
+ Hits          12709    12710    +1     
  Misses         3535     3535           
+ Partials       1407     1406    -1     

Impacted Files	Coverage Δ
.../com/alipay/sofa/rpc/context/RpcInvokeContext.java	81.48% <0.00%> (-0.93%)	:arrow_down:
...com/alipay/sofa/rpc/context/RpcRuntimeContext.java	90.12% <0.00%> (+1.23%)	:arrow_up:
...n/java/com/alipay/sofa/rpc/log/TimeWaitLogger.java	84.00% <0.00%> (+4.00%)	:arrow_up:

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