docs: Add security policy (#3217)

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- [x] Added new tests, or not needed, or not feasible
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- [x] Updated the official documentation or not needed
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SECURITY.md

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+# Security Policy
+
+The gno.land community strives to contribute toward the security of our ecosystem through internal security practices, and by working with external security researchers from the community.
+
+## Reporting a Vulnerability
+If you've identified a vulnerability, please report it through one of the following venues:
+
+* Submit an advisory through GitHub: https://github.com/gnolang/gno/security/advisories/new
+* Email security [at-symbol] tendermint [dot] com. If you are concerned about confidentiality e.g. because of a high-severity issue, you may email us for PGP or Signal contact details.
+* A security bug bounty platform for gno.land will be available Soonᵀᴹ.  You will need to report via our bug bounty platform in order to be eligible for rewards.
+
+We will respond within 3 business days to all received reports.
+
+Thank you for helping to keep our ecosystem safe!