update readme with requirements

README.md

-# nystrom-layer
+# Minimal working example of Nyström layer
 
-Minimum working example of Nystrom Layer implementation
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+Minimum working example of the Nystrom Layer implementation.
+
+## Requirements
+
+Those requirements versions may not be absolutely necessary but they are the versions
+used while writing this simple example
+
+- `python=3.6` 
+- `keras=2.2.4`
+- `tensorflow=1.13.1`
+- `numpy=1.16.4`
+
+## Files
+
+- `nystrom_layer.py`: the minimal working example that you can just execute as it is.
+
+- `keras_kernel_functions.py`: some common kernel functions implemented in keras.
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