[Documentation] Add explanation of Setup() and TearDown()

[siekermantechnology]

The Run() (or TestRunner__Run() function in fw_testRunner.brs has support for a SetUp() and TearDown(). It would be good if explanation of that is added to the documentation.

[RokuRnD]

@siekermantechnology thanks for the request. Done in https://github.com/rokudev/unit-testing-framework/releases/tag/v.1.3.0