Add a contributing guide

[MarkEWaite]

Add a contributing guide

Contributing guide makes it easier for new contributors to help with the plugin.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[gounthar]

Looks good to me. 👍
That one was much needed, thanks.