Create security policy following github's template (#5070)

Instructions copied from https://github.com/mongodb/mongo-ruby-driver#security-issues.

SECURITY.md

@@ -0,0 +1,19 @@
+# Security Policy
+
+## Supported Versions
+
+Use this section to tell people about which versions of your project are
+currently being supported with security updates.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 7.3     | :white_check_mark: |
+| 7.2     | :white_check_mark: |
+| 7.1     | :white_check_mark: |
+| 7.0     | :white_check_mark: |
+| < 7.0   | :x:                |
+
+## Reporting a Vulnerability
+
+Please [follow the instructions here](https://docs.mongodb.com/manual/tutorial/create-a-vulnerability-report/)
+to report a vulnerability.