Add pi-gen to tools section (thibmaek#22)

Author: Oliver2213

README.md

@@ -42,6 +42,7 @@ Contributions *very welcome* but first see [Contributing](#contributing)
 * [Etcher](https://www.etcher.io/) - An SD card burner app that is simple for end users, extensible for developers, and works on any platform.
 * [PiShrink](https://github.com/Drewsif/PiShrink/) - A bash script that automatically shrinks a pi image that will then resize to the max size of the SD card on boot.
 * [OpenVPN-Setup](https://github.com/StarshipEngineer/OpenVPN-Setup) - Shell script to set up Raspberry Pi as an OpenVPN server.
+* [pi-gen](https://github.com/RPi-Distro/pi-gen) - Tool used to create the raspberrypi.org Raspbian images. This can be used to create your own custom images with specific packages installed, etc.
 
 # Projects
 * [Mini OONTZ](https://cdn-learn.adafruit.com/downloads/pdf/mini-oontz-3d-printed-midi-controller.pdf) - 3D printed mini MIDI controller.