added testing guide and function tips

[robertswh]

Summary

Summarised today's team discussion on testing and functions

Checklists

• read new sections of CONTRIBUTING.md to ensure they are consistent with discussion at team meeting
• nothing significant is missing from the guide that was discussed

[rowanhemsi]

Good summary of our discussion and addition to the contributing guidance.
Left two suggestions on the function section but happy to approve

[rowanhemsi]

Just noticed the Check for trailing whitespaces check is failing, please can you install and run the precommit hooks locally or fix the issue manually before merging