Add a SECURITY.md file.

Change-Id: Iae6d2d1f7cfc151baf147e96e89ba079420aa2b9
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59070
Reviewed-by: Paul Wankadia <[email protected]>

Author: junyer

SECURITY.md

@@ -0,0 +1,4 @@
+To report a security issue, please use https://g.co/vulnz. We use
+https://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 https://g.co/vulnz.