docs: add security policy (#314)

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please **do NOT** raise a GitHub Issue to report a security vulnerability. This kind of issues must
+be communicated in a private manner. If you believe you have found a security vulnerability, report
+it to our dedicated email address
+[[email protected]](mailto:[email protected]). We ask that you do not use
+other channels or contact project contributors directly. Non-vulnerability related security issues
+such as new great new ideas for security features are welcome on GitHub Issues.
+
+On top of that, if you would like to provide a patch, please **do not open a pull request** as it is
+another public way to discover the vulnerability. Instead, create a commit on your fork of
+**empathyco/x** and run this command:
+
+```bash
+git format-patch -1 HEAD --stdout > x_security_patch.txt
+```
+
+This command will create a file called `x_security_patch` with the content of your last commit.
+
+As a summary, send an email with:
+
+- Subject: Interface X Security Alert
+- Content: Description of the vulnerability
+- Attach the `x_security_patch` file if you have created one.
+
+Thanks for your contribution.