docs: add link to the security mailing list

Signed-off-by: Doug Davis <[email protected]>

Author: duglin

README.md

@@ -218,6 +218,10 @@ for how PR reviews and approval, and our
 [Code of Conduct](https://github.com/cloudevents/spec/blob/master/community/GOVERNANCE.md#additional-information)
 information.
 
+If there is a security concern with one of the CloudEvents specifications, or
+with one of the project's SDKs, please send an email to
+[[email protected]](mailto:[email protected]).
+
 ## Additional SDK Resources
 
 - [List of current active maintainers](MAINTAINERS.md)