docs: SECURITY.md says how to report a vulnerability

SECURITY.md

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+# Security
+
+Please report a vulnerability privately, not in a public issue:
+
+- on GitHub, with **Report a vulnerability** under the
+  [Security tab](https://github.com/karleinarb/lazysecrets/security), or
+- by email to karl-einarb@outlook.com.
+
+Say what's affected, how to reproduce it and the version
+(`lazysecrets --version`). Fixes go into the next release, and only the
+latest release gets them.