README.md: Use maintainers list for reporting security issues
README.md: Use maintainers list for reporting security issues

Signed-off-by: Stephen Augustus <f96984fef9637085f0559f1484dfd4e99545cd6e@vmware.com>

coreos/dex,ericchiang/dex,coreos/dex,ericchiang/dex,ericchiang/dex,coreos/dex