Update README.md

Author: dnrobin

README.md

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 # awg-matlab
-AWG model with Matlab
+Start with the examples in `examples/` directory.
+
+## Important notes
+* All lengths are [µm] and frequencies are [THz] by default. This means, for example, that 1550nm wavelength will be written 1.55 and 193.5THz is simply 193.5.
+
+* The model currently only handles strip waveguides with constant 220nm height, widths may vary from 350nm to 2µm, freespace wavelength must be within 1.4 to 1.6µm, core material is Silicon.
+
+* Spot size gaussian approximation is used by default to calculate strip waveguide profiles. It is possible to use real modes by including the option `‘RealModes’, true` in the transfer function script inputs (see example 2).
+
+* When running example scripts, make sure the active directory is the root AWG model directory and not the examples directory.