add README for SageMaker Spark

Author: andremoeller

sagemaker-spark/README.md

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+# Amazon SageMaker Examples
+
+## Amazon SageMaker using SageMaker Spark
+
+These examples demonstrate using SageMaker Spark, which allows you to interleave Apache Spark code with Amazon SageMaker.
+
+See [SageMaker Spark](https://github.com/aws/sagemaker-spark) for more information.
+
+- [K-Means Clustering on MNIST with SageMaker Spark](pyspark_mnist)