Add SECURITY.md

[somiaj]

This is an issue for #2887. The actual file provided will need to be updated for our use case, so making this an issue until we get around to adding the file.

# Security Policy

## Supported Versions

<!-- Please update the table below to reflect for which versions this project will receive security reports for -->

| Version | Supported          |
| ------- | ------------------ |
| [`2.20`](https://github.com/openwebwork/webwork2/releases/tag/WeBWorK-2.20)    | :white_check_mark: |
| [`2.19`](https://github.com/openwebwork/webwork2/releases/tag/WeBWorK-2.19)    | :white_check_mark: |
| [`2.18`](https://github.com/openwebwork/webwork2/releases/tag/WeBWorK-2.18)    | :white_check_mark: |
| <=[`2.17`](https://github.com/openwebwork/webwork2/releases/tag/WeBWorK-2.17)  | :x:                |

## Reporting a Vulnerability

**Please DO NOT report any security vulnerabilities through GitHub Issues**

Rather, use the following email to report any security vulnerabilities:
thewebworkproject@gmail.com

We will respond to all security reports within `<number>` days.