Document expected size of pull requests

### What problem does your feature request solve?

Often massive pull requests are opened against the CSET repository. These pull requests are difficult to review, and can spend months without getting merged, slowing down our pace of development and preventing CSET from accessing new capabilities.

### Describe the solution you'd like

Add documentation on size of pull requests. One diagnostic per branch, which minimises the need to have complicated branch-of-branch chains. Minimise branching off of other people's branches, as they are not responsible for keeping your branch-of-branch up-to-date with expected changes. Instead, look to review their changes to get it merged into trunk.

### Describe alternatives you've considered

No file or test is named. Start by locating existing contributor or pull-request guidance in the repository, then document expectations for reasonably sized pull requests, one diagnostic per branch, and avoiding branch-of-branch dependencies. Done means the guidance is added where contributors will find it and clearly covers these practices.

Written by the indexing model from the issue text.