[Regression Testing]: Need Documentation for Adding Regression Tests

[JacksonBurns]

Motivation or Problem

Documentation for how to add a regression tests exists on RMG-tests but is out of date and not where one would expect to find it.

Desired Solution

Transfer and update the linked page to this repository.

Potential Alternatives

Inline documentation (a README in the regression testing directory) should also be added.

Additional Context

Do we actually maintain the RMG-Py wiki? It's primary purpose seems to just be to link to the docs, which are (relatively) up to date.

[JacksonBurns]

Added here.