Add PEP 207 guidance on rich comparisons to PEP 8.

pep-0008.txt

@@ -667,6 +667,21 @@ Programming Recommendations
       None was set to some other value.  The other value might have a type
       (such as a container) that could be false in a boolean context!
 
+    - When implementing ordering operations with rich comparisons, it is best to
+      implement all six operations (__eq__, __ne__, __lt__, __le__, __gt__,
+      __ge__) rather than relying on other code to only exercise a particular
+      comparison.
+
+      To minimize the effort involved, the functools.total_ordering() decorator
+      provides a tool to generate missing comparison methods.
+
+      PEP 207 indicates that reflexivity rules *are* assumed by Python.  Thus,
+      the interpreter may swap y>x with x<y, y>=x with x<=y, and may swap the
+      arguments of x==y and x!=y.  The sort() and min() operations are
+      guaranteed to use the < operator and the max() function uses the >
+      operator.  However, it is best to implement all six operations so that
+      confusion doesn't arise in other contexts.
+
     - Use class-based exceptions.
 
       String exceptions in new code are forbidden, because this language