Add Matthew Kehoe to community directory

[matthewshawnkehoe]

Please add me to the code4math community directory:

- Matthew Kehoe
    - Michigan Technological University
    - https://github.com/matthewshawnkehoe
    - mskehoe001@gmail.com
    - https://www.linkedin.com/in/matthewshawnkehoe
    - Numerical PDEs, Numerical Linear Algebra, Scientific Machine Learning