Update README.md

Add build instructions.

README.md

@@ -26,6 +26,35 @@ will be useful to someone.
 Eventually I plan to build this out by adding more sensors, a proper enclosure,
 and an external solar/battery power-source.
 
+### Build Instructions
+
+Note I'm new to `cmake` so have really just kludged this together for the moment, however this seems to work for me:
+
+1. Install/setup the Pico SDK and ensure the PICO environment vars are set up: `PICO_SDK_PATH`, `PICO_EXAMPLES_PATH`, `PICO_EXTRAS_PATH`, `PICO_PLAYGROUND_PATH` (I don't know if they're all needed)
+2. Check out the [Melopero Perpetuo LoRa](https://github.com/melopero/Melopero_Perpetuo_Lora) project.
+3. Check out this project and `cd` to the project root
+4. Edit the `CMakeList.txt` to point to the correct Melopero Perpetuo LoRa `src` path!
+5. `cd` to the `src` directory.
+6. `mkdir build`
+7. `cd build`
+8. `cmake ../.. -DPICO_PLATFORM=rp2350`
+9. `make`
+
+There should now be the files `sender.uf2` and `receiver.uf2` in the `src` directory (under the `build` directory you're currently in). Then copy these onto the boards...
+
+1. With no power (i.e. battery unplugged) plug your Melopero Perpetuo LoRa board into your computer via USB whilst holding down the "BT" button, then, for example:
+2. `sudo mount /dev/sdb1 /mnt/tmp`
+3. `sudo cp src/receiver.uf2 /mnt/tmp/`
+4. `sync`
+5. `sudo umount /mnt/tmp`
+
+In your case you may well have a different device than `/dev/sdb1` and `/mnt/tmp` is just my choice of path, this can be whatever you want.
+
+After this is run you should be able to attach to the USBTTY to view the serial output, in my case:
+
+* `minicom -D /dev/ttyACM1 -b 115200`
+
+In terms of finding the correct devices above `dmesg` is your friend.
 
 ### Disclaimer