readme: added build instructions

README.md

@@ -8,6 +8,30 @@ A firmware for the Raspberry Pi Pico W to grab data from a few environmental sen
 
 I dunno, cause I wanted to play with these sensors and keep track of the air quality in my room. But eCO2 is pretty inaccurate, and I suspect I have a fake ENS sensor because the readings seem almost random.
 
+## Instructions
+
+### Prep
+
+You need to have the pico-sdk installed. I have it installed in ~/workspace/rp2040/pico-sdk and this is currently hard-coded into CMakeFiles.txt. I'm not sure if exporting an environment variable overrides this by default, but you can always just edit it to point to wherever you have it installed.
+### Build
+
+Create a new directory in the project called build. Move into that directory and run:
+
+```
+cmake ..
+make
+```
+and if you have a picoprobe or RasPi debug probe installed (or other CMSIS-DAP compatible probe) you can then directly write to the Pico with
+```
+make flash
+```
+or, you can hold down the BOOT button on the Pico W while plugging it in. It will appear as a USB drive. Then copy the ensaht.uf2 file from the build/ directory to that USB drive. It'll take a second, then it'll disappear. You may need to manually reset the Pico (or unplug/replug it). Open a serial terminal (I used to recommend minicom until I found tio, which is much better), probably something like
+
+```
+tio /dev/ttyACM0
+```
+and it will print out sensor values. It will also log these to whatever MQTT server you have set up, assuming it was able to connect to Wi-Fi. And that's it!
+
 ## License
 
 Unlike most software I write, this is released under MIT instead of under MPL. So, follow the MIT license if you wanna reuse it!