Add instructions for submitting discussions/issues

[fredjaya]

Aim to provide guidelines for whether users and contributors should submit a GitHub issue vs. discussion.

Based off the system to be used in the workshop - most things should be first submitted as a discussion, which can then be escalated to an issue by a core dev if warranted.

Relevant: https://gavinhuttley.github.io/tib/contribute.html

[GavinHuttley]

most things should be first submitted as a discussion

If someone is sure they have identified a bug, or an error in docs, they should definitely raise an issue. For everything else, starting as a Discussion seems like a better choice.