Add more to the keyboard customization doc (still a WIP)

doc/keyboard-layout-customization.md

@@ -1 +1,28 @@
 # How to customize your keyboard layout with System76 EC
+
+## Dependencies
+* Install [sdcc](http://sdcc.sourceforge.net/): `sudo apt install sdcc`
+* Install rust nightly
+  - Install [rustup](https://rustup.rs/)
+  - Reboot
+  - Run `rustup install nightly`)
+  
+## Adding your layout
+* In `src/board/system76/{your-model}/keymap/`, copy `default.c` and rename it. I named mine `levi.c`.
+* In `src/common/include/common/keymap.h` you will find the key definitions.
+TODO: elaborate on this
+
+## Point EC at your keymap
+* In `ec/src/board/system76/{model}/board.mk` change `KEYMAP?=default` to the name of your layout.
+TODO: There's probably a better way to do this
+
+## Test build your EC
+* From the `ec` directory, run `make BOARD=system76/{your-model}` to make sure it builds correctly.
+
+## Flash your EC
+* If the test build went well, you should be ready to flash your EC:
+  - Close all running applications.
+  - Unplug everything from your laptop except the charger.
+  - Flash with `make BOARD=system76/{your-model} flash_internal`
+  - When it says, "Waiting 5 seconds for all keys to be released", it is important to not touch the keyboard or trackpad until it turns itself off.
+  - Once it shuts down, you can power it back on, and your keymap will be active.