Add some comments about req.txt, and no code for ch09

p2ch09/README.md

@@ -0,0 +1 @@
+There are no code samples for chapter 9.

requirements.txt

@@ -21,6 +21,8 @@ scikit-image>=0.15.0
 scipy
 SimpleITK
 tensorflow>=1.14,<2
+
+# The book was written targeting PyTorch 1.4, but everything in it should work fine with more up-to-date versions.
 torch==1.4
 
 # future is here due to a missing dep in caffe2