docs: add security policy

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.96%. Comparing base (f7f4f91) to head (831e985).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #58 +/- ## ======================================= Coverage 94.96% 94.96% ======================================= Files 50 50 Lines 4407 4407 ======================================= Hits 4185 4185 Misses 222 222 ```

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Bencher

Report	Thu, March 21, 2024 at 18:03:11 UTC
Project	theseus-rs/rsql
Branch	security-policy
Testbed	localhost

Benchmark	Latency	Latency Results nanoseconds (ns)
version	➖ (view plot)	44413000.000

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