Add CONTRIBUTING

[matterhorn103]

Things to make a note of:

• Use of uv and ruff
• Policy of avoiding external dependencies unless import is optional for an interface OR:
  • it does something complex or specialist AND
  • it does it faster than we could do with our own implementation AND
  • future maintenance can be relied upon
• Derived units should be defined in terms of SI base units where possible
• Repeating decimals in quantity definitions are given to 20 sf