doc: add very basic Security Policy document (libuv#4290)

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Currently, we are providing security updates for the latest release in the v1.x series:
+
+| Version | Supported          |
+| ------- | ------------------ |
+| Latest v1.x  | :white_check_mark: |
+
+## Reporting a Vulnerability
+
+If you believe you have found a security vulnerability in `libuv`, please use the [GitHub's private vulnerability reporting feature](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing-information-about-vulnerabilities/privately-reporting-a-security-vulnerability#privately-reporting-a-security-vulnerability) in the [libuv repository](https://github.com/libuv/libuv) to report it to us.
+
+This will allow us to assess the risk, and make a fix available before we add a bug report to the GitHub repository.
+
+Please do:
+
+* Provide as much information as you can about the vulnerability.
+* Provide details about your configuration and environment, if applicable.
+
+Please do not:
+
+* Post any information about the vulnerability in public places.
+* Attempt to exploit the vulnerability yourself.
+
+We take all security bugs seriously. Thank you for improving the security of `libuv`. We appreciate your efforts and responsible disclosure and will make every effort to acknowledge your contributions.