Document the pull request labels release notes are generated from

[oneleggedswede]

.github/release.yml sorts merged pull requests into release note categories by label, but the contributing guide said nothing about labels, so a pull request opened by following it arrives unlabelled and is filed under "Other Changes".

Adds a Labels section between Guidelines and Setup, and points step 4 of Process at it.

• A table of the seven labels a contributor picks from, each with the case it covers.
• A note that a pull request is listed once, under the first category it matches, so breaking plus enhancement appears under Breaking Changes.
• The calls that are less obvious: a test or CI fix is maintenance rather than bug, an incorrect doc is documentation rather than bug, a refactor stays maintenance even when it edits resources/views, and breaking applies before 1.0.

duplicate is left out. It is in the exclude list, but it is a triage label a maintainer applies when closing something rather than a call a contributor makes on their own pull request.

The four labels this documents that the repository was missing — breaking, dependencies, maintenance, skip-changelog — now exist, so every label named here is selectable.

Documentation only. No change to src/ or resources/.