Documentation: Add RunningOnRaspberryPi page

This is a first draft targeted for existing developers.

Documentation/RunningOnRaspberryPi.md

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+# Running Serenity on Raspberry Pi
+
+## NOTE
+
+This is for development purposes only - Serenity doesn't currently boot on Rasperry Pi! Use this guide if you want to set up a development environment.
+
+Currently only UART output is supported, no display.
+
+## Running in QEMU
+
+### Step 1: Setup Serenity
+
+Please follow [build instructions](BuildInstructions.md) to download and build Serenity. Make sure everything builds successfully for x86.
+
+### Step 2: Build Aarch64 toolchain
+
+Use following the command to build the toolchain for Aarch64:
+
+```console
+Meta/serenity.sh rebuild-toolchain aarch64
+```
+
+### Step 3: Build and run in emulator
+
+Use the following command to build and run the Aarch64 kernel:
+
+```console
+Meta/serenity.sh run aarch64
+```
+
+It should build Serenity and open a QEMU window, similar to the x86 version. You should see some messages in the terminal.
+
+## Running on real hardware using an SD Card
+
+### Step 0: Download and run Raspberry Pi OS from an SD Card
+
+This step is needed because the original firmware files need to be present on the SD Card when booting Serenity. It will also help with the UART setup.
+
+### Step 1: Connect your Raspberry Pi to your PC using a UART cable
+
+Please follow one of the existing guides (for example [here](https://scribles.net/setting-up-serial-communication-between-raspberry-pi-and-pc)) and make sure UART is working on Raspberry Pi OS before proceeding.
+
+### Step 2: Mount SD Card and modify config.txt file on Boot/ partition
+
+Add the following lines:
+
+```
+enable_uart=1
+arm_64bit=1
+# kernel=serenity.img
+```
+
+The last line is optional and specifies which kernel to use. You can either replace the default `kernel8.img` file with the Serenity kernel or use a custom file name to be able to easily switch between kernels.
+
+### Step 3: Copy Serenity kernel to SD Card
+
+`kernel8.img` build artifact can be found in `Build/aarch64/Kernel/Prekernel/` directory. Copy it to the main directory on `Boot/` partition, next to the `config.txt` file. You can either replace the original file or use another name (see above).
+
+### Step 4: Put the SD Card in the Raspberry Pi and power on
+
+You should start seeing some messages in your UART terminal window.
+
+## Running on real hardware using network (Raspberry Pi 3)
+
+### Prerequisites
+
+There are multiple ways to set up your network. The easiest way is a direct connection between the Raspberry Pi and your PC. To achieve this your PC has to have an Ethernet port.
+
+Here's the [Raspberry Pi Documentation](https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#debugging-network-boot-mode) on booting from the network.
+
+### Step 1: Make sure OTP mode is enabled on the board 
+
+This is enabled by default on Raspberry Pi 3+. For the previous boards please see the section [Debugging Network Boot Mode](https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#debugging-network-boot-mode) of the Raspberry Pi documentation.
+
+### Step 2: Copy all files from the original SD Card to your PC
+
+This directory will serve as a TFTP server, sending files to the Raspberry Pi when requested.
+
+### Step 2: Set up network interface
+
+Switch the network interface to static mode (static IP) and disable the firewall.
+
+### Step 3: Set up network services
+
+Booting Raspberry Pi requires DHCP and TFTP servers.
+
+On Windows, you can use the [Tftpd32](https://bitbucket.org/phjounin/tftpd64/src/master/) program.
+
+Example configuration for DHCP:
+
+![](Tftpd32_Dhcp.png)
+
+Make sure you **disable** the `Ping address before assignment` option.
+
+Example configuration for TFTP:
+
+![](Tftpd32_Tftp.png)
+
+The only option worth noting is `Base Directory` which should contain the files from the SD Card.
+
+### Step 4: Power up the Raspberry Pi
+
+Remove the SD Card, connect an Ethernet cable between the Raspberry Pi and your PC and power on the board.
+
+After 5-10 seconds you should see files being served by the TFTP server:
+
+![](Tftpd32_Serving.png)
+
+The system should boot normally as it would from the SD Card.
+
+### Step 5: Modify config.txt and copy Serenity kernel
+
+Similarly to booting from SD Card (see above), modify `config.txt` and copy the Serenity kernel to the TFTP directory.
+
+### Step 6: Reset Raspberry Pi
+
+You should start seeing some Serenity messages in your UART terminal window.