Create SECURITY.md (#727)

Author: smyrick

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We will support the current major release as well as the previous major release branch for all critical secrity fixes, while still following [semantic version standards](https://semver.org/).
+
+This means if the latest version is `4.x.x`, we will add a hot fix to the `3.x.x` branch, but not the `2.x.x` branch or anything older.
+
+## Reporting a Vulnerability
+
+If there is a published security issue with one of our dependencies that has a fix released, please create an issue or pull request with the required change.
+
+Please report any other security issues by sending an email to [email protected].