Create SECURITY.md (#187)

Author: Kludex

SECURITY.md

@@ -0,0 +1,11 @@
+# Security Policy
+
+If you think you have identified a security issue with `python-multipart`, **do not open a public issue**.
+
+To responsibly report a security issue, please navigate to the Security tab for the repo and click "Report a vulnerability."
+
+![Screenshot of repo security tab showing "Report a vulnerability" button](https://github.com/encode/.github/raw/master/img/github-demos-private-vulnerability-reporting.png)
+
+Be sure to include as much detail as necessary in your report. As with reporting normal issues, a minimal reproducible example will help the maintainers address the issue faster.
+
+Thank you.