updated README.md

README.md

@@ -1,14 +1,16 @@
 # Frequency-Dependent F-Number for Coherent Plane-Wave Compounding
 
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 Simple [MATLAB](https://mathworks.com/products/matlab.html) implementation of
 the frequency-dependent F-number
@@ -184,6 +186,7 @@ help das_pw
 
 to obtain an explanation of the input and output arguments.
 
+The typical usage is:
 
 ```matlab
 [ image, F_number_values ] = das_pw( positions_x, positions_z, data_RF, f_s, e_theta, element_width, element_pitch, ( 1 - N_elements ) / 2, [ f_lb, f_ub ], c_ref, N_samples_shift, window, F_number);