Introduce SECURITY.md

[Bertk]

This pull request introduces a new SECURITY.md file to document the project's security policy, including supported versions and guidelines for reporting vulnerabilities.

Security policy documentation:

• SECURITY.md: Added a new file outlining the supported versions of coverlet.collector, coverlet.console, and coverlet.msbuild, as well as instructions for reporting vulnerabilities.

see also #1758

>dotnet list package --vulnerable --include-transitive

The following sources were used:
   https://api.nuget.org/v3/index.json

The given project `coverlet.core` has no vulnerable packages given the current sources.
The given project `coverlet.msbuild.tasks` has no vulnerable packages given the current sources.
The given project `coverlet.core.tests` has no vulnerable packages given the current sources.
The given project `coverlet.console` has no vulnerable packages given the current sources.