add example usage to README

[jhdark]

Proposed changes

Adding an example to the README

Types of changes

What types of changes does your code introduce to foam2dolfinx?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.19%. Comparing base (754e3c4) to head (1856cf8).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #20   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files           2        2           
  Lines         107      107           
=======================================
  Hits          104      104           
  Misses          3        3           

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