Docs: Add Developer Guide for Linux Users

This patch adds a straight-forward quick start
guide for linux users to get involved with the project.

docs/software/toolchain-install.md

@@ -133,3 +133,81 @@ Don't change ``user_defines.txt`` for setting up your local configuration. Inste
     -DAUTO_WIFI_ON_INTERVAL=40
 
 Note: the `!` flag is called the removal flag. It removes other flags that completely match the text after the `!`.
+
+## Toolchain Setup (Advanced) for Linux Users
+
+!!! note "Note" 
+    These instructions are meant as a quick start guide for those who develop on Linux.
+    Typically developers on Linux don't need much of an explanation and just look for the right commands for the toolchain.
+    That's what this guide will provide, without any nudging to use Microsoft VSCode IDE.
+
+
+### Cloning the Repo
+Open a Shell (Bash) in your regular workspace and clone the repo recursively like always and change into that directory.
+It is assumed that Linux users know how to use git.
+
+```
+git clone --recursive https://github.com/ExpressLRS/ExpressLRS.git
+cd ExpressLRS
+```
+
+### PlatformIO
+PlatformIO is available as a Python package and can be installed via Pip.
+Following best practices we will create a virtual environment in the repo directory first.
+
+1. Install virtual-env (just in case it's not installed by default)
+
+```
+pip install virtual-env
+```
+
+2. Setup a virtual environment to not taint your system's python installation.
+It does not matter where you create this but it makes sense to create it in the project directory, because best practice is to have a separate venv for each python project.
+
+```
+python -m venv .venv
+source .venv/bin/activate
+```
+
+3. Install the required python packages. Empy and pexpect are listed because modules like `dronecan` fail to correctly specify
+their dependencies for pip to install them automatically.
+
+```
+pip install platformio dronecan setuptools empy==3.3.4 pexpect
+```
+
+### Building Targets using PlatformIO
+
+The platformio package comes with the [pio](https://docs.platformio.org/en/latest/core/userguide/cmd_run.html) executable.
+It is the build command of the toolchain.
+Pio expects a platformio.ini file in the directory where it is invoked, so change into src directory first.
+
+There find and open the `user_defines.txt` file in your favorite editor.
+This file contains the build flags that you know from ELRS Configurator.
+At least uncomment the line for your regulatory domain. That's the minimum requirement for the build to work.
+You may also want to set your binding phrase and everything else you would normally set in the ELRS Configurator.
+
+To verify your build environment does work, start building firmware for a common 900Mhz target. It obviously needs a corresponding regulatory domain setting.
+
+```
+cd src
+pio run --environment Unified_ESP32_900_RX_via_BetaflightPassthrough
+```
+
+At some point at the end of the build you will be asked to select some configuration between different manufacturers and products.
+This sets parameters to provision a unified software binary with information about the device hardware, e.g. if the device has a power amplifier, diversity, etc. .
+
+When you reach that point, this indicates that everything did work. You can abort with Ctrl-C and Enter now.
+
+### Finding the right configuration for your target
+
+This is the most tricky and obscure part of the ELRS project.
+Browsing through the `platformio.ini` file already gives hints about the correct environment, but typically you need to look into the [targets](https://github.com/ExpressLRS/targets) and into the [devices](https://github.com/ExpressLRS/ExpressLRS-Configurator/tree/master/devices) to back-track what the configurator would select for your device.
+Another option is to look into the logs of the ELRS Configurator after it flashed your device. You should find the call to pio together with the arguments there.
+
+### Flashing
+
+When you select `--target upload` and provide an `--upload-port`, pio will flash the device as the final step of the build, again asking you about manufacturer / device. Under the hood pio then invokes [esptool](https://github.com/espressif/esptool).
+
+```
+pio run --target upload --environment Unified_ESP32_900_RX_via_BetaflightPassthrough --upload-port /dev/ttyUSB0