Add security policy

[nobodyiam]

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.50%. Comparing base (5bddb89) to head (7e15d71).
⚠️ Report is 20 commits behind head on master.

Additional details and impacted files

@@             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     

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