Add mvn note to development/test section in README

[RyanMillerC]

Purpose

Project should call out that mvn is required in development setup.

Following dev setup instructions and running tox test will result in an error for tests/step_implementers/generate_metadata/test_maven_generate_metadata.py::TestStepImplementerMavenGenerateMetadata::test_run_step_pass if the mvn command isn't available.

E       sh.CommandNotFound: mvn

.venv/lib/python3.9/site-packages/sh.py:3457: CommandNotFound

Might save somebody time troubleshooting 🤷

Breaking?

No

[codecov[bot]]

Codecov Report

Merging #289 (e557058) into main (d0a7f54) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #289   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          100       100           
  Lines         4202      4202           
=========================================
  Hits          4202      4202           

Flag	Coverage Δ
pytests	100.00% <ø> (ø)

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[RyanMillerC]

I opened #290 that has this change and more.