Add additional model changes instruction to pull_request_template.md

[matentzn]

Resolves [#ISSUE, #ISSUE]

• docs/ have been added/updated if necessary
• make test has been run locally
• tests have been added/updated (if applicable)
• CHANGELOG.md has been updated.

If you are proposing a change to the SSSOM metadata model, you must

• provide a full, working and valid example in examples/
• provide a link to the related GitHub issue in the see_also field of the linkml model
• provide a link to a valid example in the see_also field of the linkml model
• make sure any new slot is annotated with the appropriate added_in annotation
• run SSSOM-Py test suite against the updated model

[Add a description, mentioning at least relevant #ISSUE and how it was addressed. A bulleted list of all changes performed by the PR is is helpful.]

[gouttegd]

Thanks!

[ehartley]

Looks good!