Added info about compiling on GPUs to README
Added info about compiling on GPUs to README


Former-commit-id: b0c593fc214fdb28fc4c63f85b4a24f822cdfec9
shuheik/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,clab/cnn,clab/dynet,clab/cnn,xunzhang/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,clab/dynet,clab/dynet,danielhers/dynet,clab/cnn,danielhers/dynet,clab/dynet,danielhers/dynet,clab/dynet,xunzhang/dynet,chunyang-wen/dynet,xunzhang/dynet