Create library-install.md

library-install.md

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+## Installing the PinChangeInterrupt Library
+
+This sketch version requires the library ["PinChangeInterrupt"](https://playground.arduino.cc/Main/PinChangeInterrupt) for interrupt handling.
+This is a non-standard library (it is not installed by default). Execute the following steps to install it onto your PC.
+You only need to do this once.
+
+1. Go to 'Sketch' => 'Include Library' => 'Manage Libraries...':
+
+![libray-install1](https://github.com/amunters/bitx40/blob/master/library-install1.PNG)
+
+2. The Library Manager will be started. Wait until the list of installed libraries is updated:
+
+![libray-install2](https://github.com/amunters/bitx40/blob/master/library-install2.PNG)
+
+3. In the search box, enter "pinchangeinterrupt":
+
+![libray-install3](https://github.com/amunters/bitx40/blob/master/library-install3.PNG)
+
+4. Select the libary named PinChangeInterrupt by NicoHood, then press "install":
+
+![libray-install4](https://github.com/amunters/bitx40/blob/master/library-install4.PNG)
+
+5. Wait until the installation is completed, then press "close":
+
+![libray-install5](https://github.com/amunters/bitx40/blob/master/library-install5.PNG)