Add SECURITY.md to document security policy (Qiskit#754)

This commit adds a SECURITY.md file to the repository to document the
security policy. I recently enabled the private security advisories
feature on the repository (which is a relatively new "beta" feature in
github). Since we now have a place to privately raise potential security
issues (besides email) it is good to have a documented policy on how
security vulnerabilities should be reported and our support policy for
version we will fix (which is just the latest release). As the project
matures we can adjust this policy as needed (likely to support more than
one version at a time).

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

SECURITY.md

@@ -0,0 +1,21 @@
+# Security Policy
+
+## Supported Versions
+
+Rustworkx supports one minor version release at a time both for bug and
+security fixes. For example, if the most recent release is 0.12.1 the 0.12.x
+release series is currently supported.
+
+## Reporting a Vulnerability
+
+To report vulnerabilities you can privately report a potential security issue
+via the Github security vulnerabilities feature. This can be done here:
+
+https://github.com/Qiskit/rustworkx/security/advisories
+
+Please do **not** open a public issue about a potential security vulnerability.
+
+You can find more details on the security vulnerability feature in the Github
+documentation here:
+
+https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability