Add instructions about dropping in compiled firmware on linux (#549)

* Add instructions about dropping in compiled firmware on linux

* Make it clearer in the readme that the hex file is needed whenever building from source

README.md

@@ -183,3 +183,5 @@ The tag can be pushed from your local environment, or via the ["Draft a new Rele
 ### Choosing the firmware release to be bundled with the build
 
 The CI automatically downloads a given firmware release from the [ayab-firmware repo](https://github.com/AllYarnsAreBeautiful/ayab-firmware) and packs it into the Desktop release. The name of the firmware release is chosen in [this manifest file](https://github.com/AllYarnsAreBeautiful/ayab-desktop/blob/1.0.0-dev/src/main/resources/base/ayab/firmware/manifest.txt) in the ayab-desktop repo.
+
+Whenever building from source, you should ensure that the firmware is available as a compiled hex file is available at `main/resources/base/ayab/firmware/firmware.hex`. You can either compile the firmware and upload it through other means (e.g. VSCode with PlatformIO), or compile and then drop it into the right path in the desktop directory, making it available for the flash through AYAB desktop.

TEST_GUIDE.md

@@ -6,9 +6,12 @@ The full list of builds is on the [releases](https://github.com/AllYarnsAreBeaut
 
 If you're just interested in poking around, choose the most recent build.
 
-When you click on a build you should see a list of files under the **Assets** section. Click on the correct file for your operating system. If you're on a mac, you want the **.dmg**, if you're on windows you want the **.exe**, if you're on Linux you'll have to build from source.
+When you click on a build you should see a list of files under the **Assets** section. Click on the correct file for your operating system. If you're on a mac, you want the **.dmg**, if you're on windows you want the **.exe**. Open the file you downloaded and install it the way you normally would. If you're on Linux you'll have to build from source.
+
+
+### Extra Linux setup
+Follow [instructions in the firmware repository](https://github.com/AllYarnsAreBeautiful/ayab-firmware/tree/v1.0-dev), and compile the firmware, making the compiled hex file available in your AYAB desktop directory at `src/main/resources/base/ayab/firmware/firmware.hex`. If you've compiled with PlatformIO in VSCode, you can find the hex file at `.pio/build/uno/firmware.hex` in your AYAB firmware directory. This step needs to be done if you want to use the AYAB desktop utility to flash the firmware - you can of course always flash it through PlatformIO, as well.
 
-Open the file you downloaded and install it the way you normally would.
 
 ### Extra Mac setup
 
@@ -30,7 +33,7 @@ Click **Ok** to dismiss the dialog.
 
 ## Install the firmware
 
-These releases have the corresponding firmware bundled in. You no longer need to specify which machine  you have for the firmware.
+These releases have the corresponding firmware bundled in. You no longer need to specify which machine  you have for the firmware. This is with the exception of Linux, where you need to compile and drop in the firmware to upload it through AYAB Desktop (or upload it through other means - described above in the Extra Linux setup section).
 
 Connect your AYAB hardware to your computer.