Create/Update Full OS Dependency List in README

[rquander]

A subset of dependencies for certain tests in the OpenPOWER Host OS Test Suite were not clear to the user.

The full list of OS dependencies (tools & settings) for the Test Suite must be documented in the README for user awareness.

A separate enhancement issue will be opened to track the integration of additional dependency checks within the Test Suite itself to automatically warn the user when one or more is missing.

[rquander]

Clarified Host OS and Guest VM support and created "Under Construction" sections for Host OS and Guest VM Prereqs in the README via Sept 7 Commit.

[rquander]

Currently know manual prerequisites are listed in the README along with text indicating the wrapper script's automatic tooling prereq checking and installation. As new prerequisites arise, the README will be updated.