Add documentation on how to get a PR merged

[sbryngelson]

Tracking item for adding documentation on how to get a PR merged.

• Performance

  • GPU and CPU wall clock times with and without the new feature
  • Weak scaling with the new feature

• Documentation

  • Doxygen (SHB: add link to syntax)

• Tests

  • Add relevant tests to the test suite (SHB: add link on how to do that)

• Checks

  • Add checks to the appropriate codes (preprocess, simulation, postprocess) in m_checker to be sure incompatible features are not used

• Code quality

  • Code should not be longer than is required (in, say, line count)
  • Comments that are really personal notes-to-self should be removed

[sbryngelson]

closed via #337