Update README.md

nnvm/README.md

 # mxnngraph
-Prototype of graph optimizer and construction API for next generation core engine in MXNet
+Prototype of graph optimizer and construction API for next generation core engine in MXNet and beyond
+
+## Goal
+- Construct graph easily
+- Pluggable inference, optimization algorithms
+- Can be used as input graph execution in various settings, specifically be able to support mxnet's symbolic execution API
 
 ## Related Projects
 - https://github.com/dmlc/mxnet