Update README.md (#237)
Update README.md (#237)

DOC

Expand the instructions for rolling your own TensorFlow build. 

Tested on my linux machine and assured that the warning `Your CPU supports instructions...` goes away.
tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-node,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-node,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-node,tensorflow/tfjs-node,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-node