Clarified instructions

dlib/matlab/README.txt

 This folder contains a set of tools which make it easy to create MATLAB mex
 functions.  To understand how they work, you should read the
-example_mex_function.cpp and example_mex_callback.cpp examples.
+example_mex_function.cpp, example_mex_struct.cpp, and  example_mex_callback.cpp examples.
 
 To compile them, you can use CMake.  In particular, from this folder execute
 these commands:

dlib/matlab/example.m

-% This example calls the two mex functions defined in this folder.  As you
+% This example calls the three mex functions defined in this folder.  As you
 % can see, you call them just like you would normal MATLAB functions.
 
 x = magic(3)
@@ -8,3 +8,9 @@ y = 2*magic(3)
 
 z = example_mex_callback(x, @(a)a+a)
 
+
+input = {}
+input.val = 2
+input.stuff = 'some string'
+output = example_mex_struct(input)
+