feat: add security policy

SECURITY.md

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+<img width=250px src="https://atsign.dev/assets/img/atPlatform_logo_gray.svg?sanitize=true">
+
+# Atsign Foundation Open Source Security Policies and Procedures
+
+This document outlines security procedures and general policies for the
+Atsign Foundation Open Source projects as found on
+https://github.com/atsign-foundation.
+
+  * [Reporting a Vulnerability](#reporting-a-vulnerability)
+  * [Disclosure Policy](#disclosure-policy)
+
+## Reporting a Vulnerability 
+
+The Atsign Foundation team and community take all security vulnerabilities
+seriously. Thank you for improving the security of our open source 
+software. We appreciate your efforts and responsible disclosure and will
+make every effort to acknowledge your contributions.
+
+Report security vulnerabilities by emailing the Atsign security team at:
+
+    [email protected]
+
+The lead maintainer will acknowledge your email within 24 hours, and will
+send a more detailed response within 48 hours indicating the next steps in 
+handling your report. After the initial reply to your report, the security
+team will endeavor to keep you informed of the progress towards a fix and
+full announcement, and may ask for additional information or guidance.
+
+Please report security vulnerabilities in third-party modules to the person
+or team maintaining the module.
+
+## Disclosure Policy
+
+When the security team receives a security bug report, they will assign it
+to a primary handler. This person will coordinate the fix and release
+process, involving the following steps:
+
+  * Confirm the problem and determine the affected versions.
+  * Audit code to find any potential similar problems.
+  * Prepare fixes for all releases still under maintenance. These fixes
+    will be released as fast as possible to pub.dev where applicable.