[llvm] [Offload][Conformance] Add README file (PR #155190)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 commented:

Most of this is documentation, I'd prefer that go with the `libc` documentation somewhere. Maybe @lntue knows a good place for that to live. Building instructions should link to offload / libc. This should probably just be a description of how to run the tests.

https://github.com/llvm/llvm-project/pull/155190