Merge pull request #197 from QuantEcon/ot_update

Added column major order note to OT lecture

Author: jstac

lectures/opt_transport.md

@@ -366,6 +366,17 @@ print("z:", res.x)
 print("X:", res.x.reshape((m,n), order='F'))
 ```
 
+Notice how, in the line `C_vec = C.reshape((m*n, 1), order='F')`, we are
+careful to vectorize using the flag `order='F'`.
+
+This is consistent with converting $C$ into a vector by stacking all of its
+columns into a  column vector. 
+
+Here `'F'` stands for "Fortran", and we are using Fortran style column-major order.
+
+(For an alternative approach, using Python's default row-major ordering, see [this
+        lecture by Alfred
+        Galichon](https://www.math-econ-code.org/mec-optim-b04).)
 
 **Interpreting the warning:**