docs: add security.md (#412)

docs/SECURITY.md

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+# Security Policy
+
+The go-tuf community is committed to maintaining a reliable and consistent TUF client implementation. If you believe you have identified a security issue in go-tuf's client protocol, please follow these guidelines for responsible disclosure.
+
+## Supported Versions
+
+You may report issues for the most recent version of go-tuf. We will not retroactively make changes to older versions.
+
+## Reporting a Vulnerability
+
+If you discover a potential security issue in this project we ask that you notify the go-tuf maintainers via our [vulnerability reporting form](https://forms.gle/ShM4s3mLbUAx5QHo8). At the minimum, the report must contain the following:
+* A description of the issue.
+* A specific version or commit SHA of `go-tuf` where the issue reproduces.
+* Instructions to reproduce the issue.
+
+Please do **not** create a public GitHub issue or pull request to submit vulnerability reports. These public trackers are intended for non-time-sensitive and non-security-related bug reports and feature requests. Major feature requests, such as design changes to the specification, should be proposed via a [TUF Augmentation Protocol](https://theupdateframework.github.io/specification/latest/#tuf-augmentation-proposal-tap-support) (TAP).
+
+## Disclosure
+
+This project follows a 90 day disclosure timeline.