[NFC] Add security policy (emscripten-core#19038)

Closes emscripten-core#19037

SECURITY.md

@@ -0,0 +1,16 @@
+# Security Policy
+
+If you have discovered a security vulnerability in this project, please report it
+privately. **Do not disclose it as a public issue.** This gives us time to work with you
+to fix the issue before public exposure, reducing the chance that the exploit will be
+used before a patch is released.
+
+Please submit the report as a [security bug on the Chromium tracker](https://bugs.chromium.org/p/chromium/issues/entry?template=Security%20Bug).
+
+Please provide the following information in your report:
+
+- A description of the vulnerability and its impact
+- How to reproduce the issue
+- Make it clear that it's an Emscripten bug.
+
+We ask that you give us 90 days to work on a fix before public exposure.