IVGCVSW-2115: Updated README.md
IVGCVSW-2115: Updated README.md

* Added line to README.md explaining the function of the get_compute_library.sh script

Change-Id: If6be1c64443dcd6d0a08a12485de3d38614542f0

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn