Create SECURITY.md (#782)

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The XMLSec library WILL provide security updates / fixes for the released versions for 5 years since [the day of the release](https://www.aleksey.com/xmlsec/news.html).
+After 5 years, the support MIGHT be provided on case-by-case basis.
+
+| Version   | Rekease date       | Supported          |
+| --------- | -------------------|------------------- |
+| <= 1.2.28 | April 16, 2019     | :x:                |
+| >= 1.2.29 | October 15, 2019   | :white_check_mark: |
+| >= 1.3.0  | April 12, 2023     | :white_check_mark: |
+
+## Reporting a Vulnerability
+
+Please use [GitHub private vulnerability reporting tool](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing-information-about-vulnerabilities/privately-reporting-a-security-vulnerability)
+to report any security issues or vulnerabilities.