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
`master` is now `main`, so update `check-format.sh` accordingly.
Also completely drop the Travis badge as we now use GitHub actions. There is
no replacement badge as the current action is pre-submission, not
post-submission.
