Create SECURITY.md (#133)

Author: cupicmarko

SECURITY.md

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+# Security policy
+
+If you believe you have found a security vulnerability in our code, we encourage you to report it to us as soon as possible.
+We ask that you do not publicly disclose any details of the vulnerability until we have had an opportunity to investigate and address it. 
+
+## Reporting a vulnerability
+
+To report a security vulnerability, go to [Report a vulnerability](https://github.com/NethermindEth/zinc/security/advisories/new).
+This will create a draft advisory. Please provide as much detail as possible including steps to reproduce the issue and any potential impact it may have.