Create SECURITY.md (#2933)

Summary:
Add SECURITY.md describing policy and linking to Meta Bug Bounty program.

Pull Request resolved: #2933

Reviewed By: saitcakmak

Differential Revision: D78632177

Pulled By: Balandat

fbshipit-source-id: 63fcd9b0a8dccf370c4ef5c8239530de21c98c7d

Author: Balandat

SECURITY.md

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+# Reporting and Fixing Security Issues
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+Please do not open GitHub issues or pull requests - this makes the problem immediately visible to everyone, including malicious actors. Security issues in this open source project can be safely reported via the Meta Bug Bounty program:
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+https://bugbounty.meta.com/
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+Meta's security team will triage your report and determine whether or not is it eligible for a bounty under our program.