Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We only support the latest version of PyHTML, but are open to backporting 
+security fixes to earlier versions on-request.
+
+## Reporting a Vulnerability
+
+We take the security of PyHTML very seriously. If you have discovered a 
+vulnerability in PyHTML, please disclose it responsibly.
+
+Some vulnerabilities we consider to be high-severity are:
+
+* Bugs where HTML, JS or CSS code can be embedded within PyHTML output
+  without making use of the `p.style`, p.DangerousRawHtml` or `p.script`
+  tags.
+* Bugs where the act of rendering PyHTML can trigger remote code execution
+  given seemingly-correct input (eg a `str` or descendant of `p.Tag`).
+
+You should disclose these vulnerabilities by creating a private issue on 
+the project's GitHub repo. We will aim to fix these issues as quickly as 
+possible.