add a warning about keras requirements to the docs

Author: lopuhin

docs/source/libraries/keras.rst

@@ -8,7 +8,7 @@ Keras_ is "a high-level neural networks API, written in Python and capable of ru
 Keras can be used for many Machine Learning tasks, and it has support for both popular
 and experimental neural network architectures.
 
-Note: only TensorFlow 1.x is supported, recommended Keras version is 2.3.1 or earlier.
+Note: only TensorFlow 1.x is supported, recommended Keras version is 2.3.1 or earlier, and eli5 version 0.13 or earlier, as you can't install TensorFlow 1.x on Python 3.9+ which is required for eli5 0.14+
 
 .. _Keras: https://keras.io/