Add security policy

Signed-off-by: Pedro Kaj Kjellerup Nacht <pnacht@google.com>

SECURITY.md

@@ -0,0 +1,17 @@
+# 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 me 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 by filling out
+[this form](https://github.com/lz4/lz4/security/advisories/new).
+
+Please provide the following information in your report:
+
+- A description of the vulnerability and its impact
+- How to reproduce the issue
+
+This project is maintained by a single maintainer on a reasonable-effort basis. As such,
+I ask that you give me 90 days to work on a fix before public exposure.