Update CONTRIBUTING.md

[JP-Ellis]

With PRs such as #385 and #371, the CONTRIBUTING.md is become further and further out of date. A rewrite is well and truly due once the above PRs have been merged.

[github-actions]

👋 Hi! The 'smartbear-supported' label has just been added to this issue, which will create an internal tracking ticket in PactFlow's Jira (PACT-1317). We will use this to prioritise and assign a team member to this task. All activity will be public on this ticket. For now, sit tight and we'll update this ticket once we have more information on the next steps.

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