Pull together documentation from various places and put into one document.
Includes some long-overdue documentation on how the code coverage data is processed and compressed.

Change-Id: Iddf2094aff1f3804cd1ea19414e40da3b7cae163
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46468Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>