docs: Add a security policy (#145)

Private vulnerability reporting is now enabled on the repository, so point reporters there.

Author: georgeguimaraes

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+Please report security issues privately through GitHub's private vulnerability reporting: open the [Security tab](https://github.com/sourcelevel/faraday-http-cache/security/advisories/new) of this repository and use "Report a vulnerability". Don't open a public issue or pull request for something you believe is a security problem.
+
+You'll get an acknowledgement within a week. If the report is accepted we'll work on a fix, publish a GitHub Security Advisory with credit to you, and release a fixed gem to RubyGems.
+
+## Supported versions
+
+Only the latest release on the 2.x line receives security fixes.