Update readme

Author: jgeudens

README.md

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 # What is ModbusSim
-ModbusSim is an application that can be used to test a modbus master. The application consists of 2 main set of modules. A GUI-less part that contains all logic that can also be used in automated tests. The second part is a GUI that can be used for manual testing.
+ModbusSim is a Modbus slave test application for validating Modbus master implementations.
+
+Key features:
+
+- Implements multiple configurable holding registers (values can be automatically incremented).
+- Generates a sine-wave signal as register values.
+- Can inject configurable communication errors to exercise master error handling.
 
 ![ModbusSim demo](ModbusSim.png)