Add security.md docs

[synfinatic]

Fixes: #344

[codecov]

Codecov Report

Merging #527 (4d3b119) into main (d807666) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #527   +/-   ##
=======================================
  Coverage   80.60%   80.60%           
=======================================
  Files          24       24           
  Lines        2882     2882           
=======================================
  Hits         2323     2323           
  Misses        492      492           
  Partials       67       67           

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d807666...4d3b119. Read the comment docs.