Collate information on contributing to the project into a single source of truth.

[GCHQDeveloper581]

As noted in #2333, information on how to contribute to the project is split over

• the wiki 'Contributing' page
• the main project README.md
• the main project CONTRIBUTING.md

This leads to duplication of information and updates getting out of sync.

The information should be collated into a single place and the other locations either removed or changed to a simple link to the main source of truth.