Define a PR template with checklist

[mthalman]

There should be a set of steps all contributors should following when submitting a PR. These should be defined in a PR template so that the contributor is prompted to check each step. This can include things like the following:

• Follow the best practices.
• When deleting a Dockerfile, ensure an issue is filed for updating any references to the affected tag.
• Provide a link to the issue/PR that prompted the need for the change.