Add setup section to readme to install test packages

[jkroll-deepgram]

I was getting the following error when trying to run the newly-documented test coverage command from #129:

ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: unrecognized arguments: --cov=deepgram
  inifile: None
  rootdir: /.../deepgram-python-sdk

I needed to install pytest-cov.

Updated the readme documentation to also specify that users should install pytest in order to run the tests.

We could create a top-level requirements.txt instead, but not sure if this would be undesirable to users who don't want to have external dependencies, especially for the optional purpose of running tests?

[jjmaldonis]

I added a requirements-dev.txt file in #124 so pytest-cov could be added to that and the README docs could be updated to pip install -r requirements-dev.txt (for development).