[ci skip] Create SECURITY.md (#11331)

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We generally only fully support the latest version, the same applies to exploits such as server crashes and item
+duplication bugs. In the transition period during larger Minecraft updates, we may still backport important fixes to the
+last minor or major release.
+
+## Reporting a Vulnerability
+
+For any issues that are NOT duplication bugs, server/client crashes, or otherwise serious exploits, please open an issue
+through the [Issues tab](https://github.com/PaperMC/Paper/issues).
+For exploits, please [join our Discord](https://discord.gg/papermc) and see the [#paper-exploit-report channel](https://discord.com/channels/289587909051416579/1208749386348101682) for
+further instructions.