Contributing guide instructions for allowing commits from maintainers

[dbeatty10]

resolves #913

Problem

We've had cases where "Allow edits from maintainers" is not enabled by the contributor. This means that a maintainer can't push commits and all commits need to be initiated by the contributor instead. This lowers the collaboration possibilities and can ultimately necessitate closing the original PR and opening a new one.

Solution

Provide a link within the contributing guide within the "external contributors" section that provides instructions for enabling commits from maintainers.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• [N/A] I have run this code in development and it appears to resolve the stated issue
• [N/A] This PR includes tests, or tests are not required/relevant for this PR
• [N/A] I have updated the README.md (if applicable)