Security policy

[mikofski]

Create security policy, see:

• https://docs.github.com/en/code-security/how-tos/report-and-fix-vulnerabilities/configure-vulnerability-reporting/adding-a-security-policy-to-your-repository
• https://docs.github.com/en/code-security/getting-started/quickstart-for-securing-your-repository#setting-a-security-policy

Consider enabling GH vulnerability reporting:

• https://docs.github.com/en/code-security/how-tos/report-and-fix-vulnerabilities/report-a-vulnerability/privately-reporting-a-security-vulnerability

AstroPy example: https://github.com/astropy/astropy/blob/main/SECURITY.md

Addresses: #2694