Add security policy

[nobodyiam]

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

[codecov]

Codecov Report

Merging #502 (7e15d71) into master (5bddb89) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #502      +/-   ##
============================================
- Coverage     46.52%   46.50%   -0.03%     
  Complexity      223      223              
============================================
  Files           332      332              
  Lines          9983     9983              
  Branches        958      958              
============================================
- Hits           4645     4643       -2     
- Misses         4854     4858       +4     
+ Partials        484      482       -2     

Impacted Files	Coverage Δ
...nder/manager/AsyncCommonDigestAppenderManager.java	61.70% <0.00%> (-6.39%)	⬇️
...ipay/common/tracer/core/appender/self/SelfLog.java	81.81% <0.00%> (+3.63%)	⬆️
...nt/interceptor/SofaTracerAsyncHttpInterceptor.java	100.00% <0.00%> (+11.11%)	⬆️

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