Tips for a Coarse Grid

[akleb]

Purpose

Add some documentation for trouble shooting really coarse grids that are intended to be used as a starting point for adaptive meshing.

Type of change

What types of change is it? Select the appropriate type(s) that describe this PR

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [x] Documentation update
• [ ] Maintenance update
• [ ] Other (please describe)

Testing

Explain the steps needed to test the new code to verify that it does indeed address the issue and produce the expected behavior.

Checklist

Put an x in the boxes that apply.

• [ ] I have run flake8 and black to make sure the code adheres to PEP-8 and is consistently formatted
• [ ] I have run unit and regression tests which pass locally with my changes
• [ ] I have added new tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation

[akleb]

Information from mdolab/MACH-Aero#57

[marcomangano]

This PR looks good to go, @joanibal @SichengHe can you review it and merge if ready?