Add security policy.

[dzmitryhil]

Description

Add security policy.

Reviewers checklist:

• Try to write more meaningful comments with clear actions to be taken.
• Nit-picking should be unblocking. Focus on core issues.

Authors checklist

• Provide a concise and meaningful description
• Review the code yourself first, before making the PR.
• Annotate your PR in places that require explanation.
• Think and try to split the PR to smaller PR if it is big.

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This change is 

[keyleu]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @masihyeganeh, @miladz68, @wojtek-coreum, and @ysv)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.03%. Comparing base (bd9ae79) to head (94a6c08).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #797   +/-   ##
=======================================
  Coverage   32.03%   32.03%           
=======================================
  Files         170      170           
  Lines       47462    47462           
=======================================
  Hits        15203    15203           
  Misses      29244    29244           
  Partials     3015     3015           

Flag	Coverage Δ
coreum	32.03% <ø> (ø)

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