add aws security disclaimer

provider/notification-aws/SECURITY.md

+Reporting Security Issues
+-------------------------------------------------------------------------------------------------------------------------------------------------
+We take all security reports seriously. When we receive such reports, we will investigate and
+subsequently address any potential vulnerabilities as quickly as possible. If you discover a potential
+security issue in this project, please notify AWS/Amazon Security via our [vulnerability reporting page](https://aws.amazon.com/security/vulnerability-reporting/) or
+directly via email to [AWS Security](aws-security@amazon.com). Please do NOT create a public GitLab issue in this project.
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