Create SECURITY.md (#187)

SECURITY.md

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+# Security Policy
+
+If you think you have identified a security issue with `python-multipart`, **do not open a public issue**.
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+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)
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+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.