README.md: Document that the high-level API requires at least nodejs 4.0
README.md: Document that the high-level API requires at least nodejs 4.0

Fixes gh-32

zqzhang/iotivity-node,zqzhang/iotivity-node,zqzhang/iotivity-node,zqzhang/iotivity-node,zolkis/iotivity-node,zolkis/iotivity-node,zolkis/iotivity-node,zqzhang/iotivity-node,zolkis/iotivity-node,zolkis/iotivity-node