Create repo security policy

[edhamiltonHO]

To meet community standards

[edhamiltonHO]

To include:

• - creating a suitable contact group for reports to flow to
• - creating a lightweight process to ensure reports are addressed
• - create an issue template or other button to report an issue
• - including a security policy on the repo - as in https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository
• created new issue to cover later Create security.txt #191 - consider the inclusion of a /.well-known/security.txt file - as in https://securitytxt.org/
• - update contributing.md to point to how to report a security issue

[keithkennedyHO]

Once we create the SECURITY.md file, a link to the security will display when a new issue is created and also on the sidebar for the repo.

This will address:

• create an issue template or other button to report an issue