Add security policy (#996)

diff --git a/SECURITY.md b/SECURITY.md
new file mode 100644
index 0000000..c2a44c6
--- /dev/null
+++ b/SECURITY.md
@@ -0,0 +1,6 @@
+### Reporting
+
+To report a security issue, please use [https://g.co/vulnz](https://g.co/vulnz).
+We use g.co/vulnz for our intake, and do coordination and disclosure here on
+GitHub (including using GitHub Security Advisory). The Google Security Team will
+respond within 5 working days of your report on g.co/vulnz.