Fill in the placeholder readme with some basic information on building and
running the project. Information on the conformance submission process and
contributing are also included.

Should help close a few issues referenced in
https://github.com/KhronosGroup/OpenCL-CTS/issues/1096

I don't think this is all the information we want, but is a starting point
from which we can progress. For example, adding the android build instructions
from https://github.com/KhronosGroup/OpenCL-CTS/pull/1021