Update CONTRIBUTING.md with testing requirements

[davidassigbi]

• Include guidance for molecule testing with verify.yml
• Include how to test for all the supported OS.
  • Either locally by using the list of supported from https://github.com/claranet/ansible-role-postgresql/blob/main/.github/workflows/molecule.yml
  • Use act
  • or Relying on the CI
• molecule[docker] changed to molecule-plugins[docker]

Context

The contribution documentation might not be lacking some steps for new contributors.

Your Environment

• Version used: 1.0.0