[Doc] Add readme to mxnet gat (#300)

* readme

* Update and rename README to README.md

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examples/mxnet/README.md

 # Model Examples using DGL (w/ MXNet backend)
 
+use `DGLBACKEND=mxnet` to use MXNet as DGL's backend
+
+## Examples:
+
+```
+DGLBACKEND=mxnet python gcn_batch.py --dataset cora
+DGLBACKEND=mxnet python gat_batch.py --dataset cora
+```
+
 Each model is hosted in their own folders. Please read their README.md to see how to
 run them.
 

examples/mxnet/gat/README.md

+Graph Attention Networks (GAT)
+============
+
+- Paper link: [https://arxiv.org/abs/1710.10903](https://arxiv.org/abs/1710.10903)
+- Author's code repo:
+  [https://github.com/PetarV-/GAT](https://github.com/PetarV-/GAT).
+
+Note that the original code is implemented with Tensorflow for the paper.
+
+
+## Usage (make sure that DGLBACKEND is changed into mxnet)
+```bash
+DGLBACKEND=mxnet python gat_batch.py --dataset cora --gpu 0 --num-heads 8
+```