Adopt a basic security policy (#2040)

This informs users to submit security reports through GitHub's private vulnerability mechanism.

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the latest version of django-debug-toolbar [![PyPI version](https://badge.fury.io/py/django-debug-toolbar.svg)](https://pypi.python.org/pypi/django-debug-toolbar) is supported.
+
+## Reporting a Vulnerability
+
+If you think you have found a vulnerability, and even if you are not sure, please [report it to us in private](https://github.com/django-commons/django-debug-toolbar/security/advisories/new). We will review it and get back to you. Please refrain from public discussions of the issue.