Update CONTRIBUTING.md

[kimadactyl]

User story

As a {person} I need {to be able to do} so that {purpose}.

Acceptance criteria

• How to join the dev team
• Component guidelines
• Code environment and setup
• Pointers to various key readmes
• PlaceCal handbook

Implementation notes & questions

Implementation plan

To be written by the developer

[kimadactyl]

There's some examples here, not a quick job unfortch: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors#examples-of-contribution-guidelines

[katjam]

I started making some notes here a long time ago but we've never prioritised - might be useful to do this at an organisation level where appropriate.

https://www.notion.so/gfsc/Update-our-repo-docs-especially-Issue-PR-templates-a81771e1b7414397b45068319c7e738b?pvs=4