Update contributing doc: PRs must be associated with an issue

[codegold79]

Description

Include a summary of the change. If this change fixes an issue, indicate that on the Fixes line. Briefly include
relevant context and motivation.

It wasn't clear before that issues are required for every PR submitted, even for trivial PRs like typo/copyright year edits. Make it clear that all PRs need to either fix or update an issue.

Updates #75

Change Details

Check all that apply:

• New feature
• Bug fix
• Non-code change
• Non-breaking change
• Breaking change
• Requires a documentation change

Release Note

How Has This Been Tested?

Describe any testing done to verify changes. Provide enough detail that tests can be reproduced.

Checklist:

• My code follows the Contribution and Style guidelines of this project
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• I have added test details that prove my fix is effective or that my feature works