Create security policy.

Create security policy.

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Maintaining multiple versions of the library is quite time consuming, so
+the majority of the effort is focused on the latest major release.
+
+If you do require a version outside of this chart updated with patch fix,
+please [contact me](mailto:github@ricmoo.com).
+
+| Version | Supported                                  | Initial Release   |
+| ------- | ------------------------------------------ | ----------------- |
+| 5.0.x   | :white_check_mark:                         | 2020-06-12        |
+| 4.0.x   | :white_check_mark: (security patches only) | 2018-10-01        |
+| 3.0.x   | :x:                                        | 2018-03-05        |
+| 2.2.x   | :x:                                        | 2018-01-11        |
+| 2.1.x   | :x:                                        | 2017-05-22        |
+| 2.0.x   | :x:                                        | 2017-04-05        |
+| 1.0.x   | :x:                                        | 2016-08-23        |
+| 0.0.x   | :x:                                        | 2016-07-14        |
+
+
+## Reporting a Vulnerability
+
+If you identify a security vulnerability with this library (or any dependency),
+please do not hesitate to contact [github@ricmoo.com](mailto:github@ricmoo.com)
+immediately.
+
+I try to respond within the same day and will address any concern as quickly
+as possible (including code fixes and publishing to NPM).
+
+Any vulnerability will also be published to this file, along with credits,
+pertinent information and links to fixes.