Add information about extra CMake build flags to README.md
Add information about extra CMake build flags to README.md

BUG=aomedia:1296

Change-Id: If9f944b58f23cdb71f919bd391f6b37e27b271f1

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom