update README

Author: zingale

lectures/11-machine-learning/README

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+Note: on older machines, tensorflow generates an illegal instruction
+and crashes python on import.  The issue is the CPU instructions it
+was compiled with.  The solution seems to be to drop down to
+tensorflow 1.5:
+
+https://github.com/tensorflow/tensorflow/issues/17411
+
+On my system, I need to make sure I got numpy from pip (instead of the
+Fedora package manager).