Update README

[naveen-rn]

README is not updated yet. We should update it with difference between OpenSHMEM build and Cray SHMEM build. OpenSHMEM build only tests the specification specific features and Cray SHMEM build and tests Cray specific extensions.

Another problem is that, we should keep it updated with Cray specific changes. At present, the test suite is tested against Cray-SHMEM/7.5.5. It works!! <RAISE BUG report if it doesn't>