Create SECURITY.md (#65)

Author: joelweinberger

SECURITY.md

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+# Security Policy
+
+The UMA project takes security very seriously. We appreciate responsible
+disclosure of security issues and will endeavor to acknowledge your
+contributions.
+
+
+## Reporting a Vulnerability
+
+If you believe you have found a security issue or problem, please report
+it through the GitHub Security Advisory
+["Report a Vulnerability" tab](https://github.com/uma-universal-money-address/uma-python-sdk/security/advisories/new).