Update docs - add compilation instructions

Author: PaulZC

docs/assets/img/Download_Zip.png

@@ -439,3 +439,73 @@ The **[/Firmware/Binaries](https://github.com/sparkfun/SparkFun_GNSSDO/tree/main
 
 You can update or reload the firmware using the [SparkFun RTK Firmware Uploader](https://github.com/sparkfun/SparkFun_RTK_Firmware_Uploader).
 
+## :fontawesome-solid-screwdriver-wrench: Compiling Firmware
+
+The GNSSDO Firmware can be compiled locally using Docker:
+
+### Clone, fork or download the GNSSDO repo
+
+To build the GNSSDO Firmware, you obviously need a copy of the source code.
+
+Click on the icon below to download a copy of the main (released) branch:
+
+[![Download ZIP](./assets/img/Download_Zip.png)](https://github.com/sparkfun/SparkFun_GNSSDO/archive/refs/heads/main.zip "Download ZIP (main branch)")
+
+For the real Wild West experience, you can also download a copy of the `release_candidate` code branch. This is where the team is actively changing and testing the code, before it becomes a full release. The code there will _usually_ compile and will _usually_ work, but we don't guarantee it! We may be part way through implementing some breaking changes at the time of your download...
+
+[![Download ZIP - release candidate](./assets/img/Download_Zip.png)](https://github.com/sparkfun/SparkFun_GNSSDO/archive/refs/heads/release_candidate.zip "Download ZIP (release_candidate branch)")
+
+### Install Docker Desktop
+
+* **(Optional)** Head to [Docker](https://www.docker.com/) and create an account. A free "Personal" account will cover occasional compilations of the firmware
+* Download and install [Docker Desktop](https://docs.docker.com/get-started/get-docker/) - there are versions for Mac, Windows and Linux. You may need to restart to complete the installation.
+* Run the Desktop
+    * You don't _need_ to have an account and you don't _need_ to be signed in
+    * You only need to be signed in if you want to store or share your Container on Docker Hub
+    * If you don't sign in, Docker Desktop will run in Personal mode - which will cover local compilations of the firmware
+* On Windows, you may see an error saying "**WSL needs updating** Your version of Windows Subsystem for Linux (WSL) is too old". If you do:
+    * Open a command prompt
+	* Type `wsl --update` to update WSL. At the time of writing, this installs Windows Subsystem for Linux 2.6.1
+	* Restart the Docker Desktop
+* If you are using Docker for the first time, the "What is a container?" and "How do I run a container?" demos are useful - _but not essential_
+    * On Windows, you may want to give Docker Desktop permission to access to your Network, so it can access (e.g.) HTML ports
+	* You can Stop the container and Delete it when you are done
+* You may want to prevent Docker from running when your machine starts up
+    * Uncheck "Start Docker Desktop when you sign in to your computer" in the Desktop settings
+
+### Using Docker to create the firmware binary
+
+* **Make sure you have Docker Desktop running.** You don't need to be signed in, but it needs to be running.
+* Open a Command Prompt and `cd` into the SparkFun_GNSSDO folder
+* Check you are in the right place. Type `dir` and hit enter. You should see the following files and folders:
+
+```
+    .gitattributes
+    .github
+    .gitignore
+	CONTRIBUTING.md
+    docs
+    Firmware
+```
+
+* `cd Firmware` and then `dir` again. You should see:
+
+```
+    app3M_fat9M_16MB.csv
+	Binaries
+    compile_with_docker.bat
+    Dockerfile
+    GNSSDO_Firmware
+	Utils
+```
+
+* The file that does most of the work is the `Dockerfile`
+
+* Run `compile_with_docker.bat` to compile the firmware. It does everything for you
+
+* Type `dir` again. Hey presto! You have your newly compiled firmware binary!
+
+You can then use (e.g.) the [SparkFun RTK Firmware Uploader](https://github.com/sparkfun/SparkFun_RTK_Firmware_Uploader) to upload the binary onto the ESP32.
+
+You may find the `.elf` file useful if you are trying to debug the code with [me-no-dev's Exception Decoder](https://github.com/me-no-dev/EspExceptionDecoder).
+