Add SECURITY.md (#2349)

* Add SECURITY.md

Content is as in other key repositories, eg. mattermost-mobile.

* Update SECURITY.md

Updated as per review, thanks for spotting this!

Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

SECURITY.md

@@ -0,0 +1,25 @@
+Security
+========
+
+Safety and data security is of the utmost priority for the Mattermost community. If you are a security researcher and have discovered a security vulnerability in our codebase, we would appreciate your help in disclosing it to us in a responsible manner.
+
+Reporting security issues
+-------------------------
+
+**Please do not use GitHub issues for security-sensitive communication.**
+
+Security issues in the community test server, any of the open source codebases maintained by Mattermost, or any of our commercial offerings should be reported via email to [responsibledisclosure@mattermost.com](mailto:responsibledisclosure@mattermost.com). Mattermost is committed to working together with researchers and keeping them updated throughout the patching process. Researchers who responsibly report valid security issues will be publicly credited for their efforts (if they so choose).
+
+For a more detailed description of the disclosure process and a list of researchers who have previously contributed to the disclosure program, see [Report a Security Vulnerability](https://mattermost.com/security-vulnerability-report/) on the Mattermost website.
+
+Security updates
+----------------
+
+Mattermost has a mandatory upgrade policy, and updates are only provided for the latest release. Critical updates are delivered as dot releases. Details on security updates are announced 30 days after the availability of the update.
+
+For more details about the security content of past releases, see the [Security Updates](https://mattermost.com/security-updates/) page on the Mattermost website. For timely notifications about new security updates, subscribe to the [Security Bulletins Mailing List](https://about.mattermost.com/security-bulletin).
+
+Contributing to this policy
+---------------------------
+
+If you have feedback or suggestions on improving this policy document, please [create an issue](https://github.com/mattermost/focalboard/issues/new/choose).