Missing offline/spirv documentation

Split out from #689 following discussion with @gwawiork and @kpet, relates to #696

There is currently a lack of documentation on how to run the conformance tests using the unified approach to offline/spirv compilation to be addressed. Ideally this would include:

* Links to files in the repository containing relevant information, code, or data
* Requirements for running the conformance tests in offline compilation mode
* Requirements for running the conformance tests in spirv compilation mode
* Outline of the steps required to run the conformances tests using offline/spirv compilation mode

Read issue #689 and related issue #696 first, then inspect the repository's offline and SPIR-V conformance-test materials. Document links to relevant repository content, prerequisites for each compilation mode, and the steps to run both modes. Done means a newcomer can follow the documented procedure.

Written by the indexing model from the issue text.