Use CONTRIBUTING.md to follow standard for most SCM systems.

What's this PR do?

Makes contribution guidelines available to developers that visit the project on Gitlab.

How should this be manually tested?

After a successful merge, the "Add Contribution Guideline" (see pic below) will change to "View.."

Any background context you want to provide?

Gitlab searches to see if CONTRIBUTING.* file exists in root of repository and adds it to the project home screen.

What are the relevant tickets?

No tickets were open, how-ever I felt this is needed.

Screenshot