Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Mastodon.py makes an effort to always be as backwards-compatible as possible so that you can update to the newest 
+version without causing compatibility issues. As such, we're not generally going to backport any possible 
+security-related fixes to older versions - the supported version is the latest one.
+
+## Reporting a Vulnerability
+
+If you find a security vulnerability that you think is critical enough to warrant such caution, please 
+feel free to report it privately to halcy+mastopysec@halcy.de . I will try to respond as quickly as possible and
+work through it with you.
+
+A possible example of such a vulnerability would be a way for a malicious server instance to overwrite local files, 
+or execute code on a client. A *non-example* would be a vulnerability in Mastodon itself - please report these to
+Mastodon, not here, Mastodon.py does not *depend* on server software and as such is not transitively vulnerable.