ARROW-3825: [Python] Document how to run the Python unit tests in python/README.md
ARROW-3825: [Python] Document how to run the Python unit tests in python/README.md

Author: Wes McKinney <wesm+git@apache.org>

Closes #2983 from wesm/python-readme-unit-testing and squashes the following commits:

fbdd23cc <Wes McKinney> Fix typos. Be more explicit about what is an in place build
b6e304f6 <Wes McKinney> Document how to run the Python unit tests in python/README.md

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