Add issue management info to CONTRIBUTING

[carreter]

Changes in this PR

Document the workflow changes discussed in #346

Why are you making these changes?

Sets up a standard for issue management, which will be important as we get more of em.

Are any changes breaking? (IMPORTANT)

No

Pre-merge checklist

All of these must be satisfied before this PR is considered
ready for merging. Mergeable PRs will be prioritized for review.

• New packages/exported functions have docstrings.
• New/changed functionality is thoroughly tested.
• New/changed functionality has a function giving an example of its usage in the associated test file. See primers/primers_test.go for what this might look like.
• Changes are documented in CHANGELOG.md in the [Unreleased] section.
• All code is properly formatted and linted.
• The PR template is filled out.