Add in the faq a section on small float variation

doc/faq.txt

@@ -25,6 +25,20 @@ Python requires that *__len__* returns an integer, yet it cannot be done as Thea
 This error message cannot be made more explicit because the relevant aspects of Python's
 internals cannot be modified.
 
+Output slight numerical difference
+----------------------------------
+
+Sometimes when you compare the output of Theano between different
+Theano flags, Theano version, CPU and GPU or with other software like
+NumPy, you will see small numerical difference.
+
+This is normal. Float number are approximation or real numbers. This
+cause that many small change like doing a+(b+c) vs (a+b)+c can give
+small difference of value. This is normal. For more detail, see: `What
+Every Computer Scientist Should Know About Floating-Point Arithmetic
+<https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html>`_.
+
+
 
 Faster gcc optimization
 -----------------------