add readme to customop

scala-package/examples/src/main/scala/org/apache/mxnetexamples/customop/README.md

@@ -0,0 +1,22 @@
+# Custom Operator Example for Scala
+This is the example using Custom Operator for type-safe api of Scala.
+In the example, a `Softmax` operator is implemented to run the MNIST example.
+
+There is also an example using RTC. However, the rtc module is depreciated and no longer can be used. Please contribute to use CudaModule operator to replace the rtc.
+
+## Setup
+### Download the source File
+```$xslt
+https://s3.us-east-2.amazonaws.com/mxnet-scala/scala-example-ci/mnist/mnist.zip
+```
+### Unzip the file
+```$xslt
+unzip mnist.zip
+```
+### Arguement Configuration
+Then you need to define the arguments that you would like to pass in the model:
+```$xslt
+--data-path <location of your downloaded file>
+```
+
+you can find more in [here](https://github.com/apache/incubator-mxnet/blob/scala-package/examples/src/main/scala/org/apache/mxnet/examples/customop/ExampleCustomOp.scala#L218-L221)