feat: added security policy for github

.github/SECURITY.md

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+# Reporting a security vulnerability
+
+NodeBB's security policy is based around a private bug bounty program. Users are invited to explore NodeBB for vulnerabilities, and report them to the NodeBB team so that they can be patched.
+
+If you have found a security vulnerability, **do not post it onto our GitHub tracker**. Some security vulnerabilities are quite severe and discretion is recommended. Email the NodeBB Security Team at security@nodebb.org, instead.
+
+# Bug Bounty Program
+
+Security vulnerability reports may be eligible for a bounty based on severity and confirmation from NodeBB team members. For full details regarding our bug bounty program, including the bounty amounts, please consult the following page: https://blog.nodebb.org/bounty