doc: add security policy

SECURITY.md

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+# Security Policies and Procedures
+
+This document outlines security procedures and general policies for the Express
+project.
+
+  * [Reporting a Bug](#reporting-a-bug)
+  * [Disclosure Policy](#disclosure-policy)
+  * [Comments on this Policy](#comments-on-this-policy)
+
+## Reporting a Bug
+
+The Express team and community take all security bugs in Express seriously.
+Thank you for improving the security of Express. We appreciate your efforts and
+responsible disclosure and will make every effort to acknowledge your
+contributions.
+
+Report security bugs by emailing the lead maintainer in the Readme.md file.
+
+To ensure the timely response to your report, please ensure that the entirety
+of the report is contained within the email body and not solely behind a web
+link or an attachment.
+
+The lead maintainer will acknowledge your email within 48 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.
+
+Report security bugs 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 npm.
+
+## Comments on this Policy
+
+If you have suggestions on how this process could be improved please submit a
+pull request.