Add instructions on how to build from the commandline

README.md

@@ -65,6 +65,14 @@ systemctl start LamyEraser.service
 When you press the button on the Lamy Pen, an input event with code BTN_TOOL_RUBBER is sent into dev/input/event1. Essentially, this tricks the reMarkable into
 thinking you are using the eraser side of the Marker Plus.
 
+# How to build
+
+* Download the latest toolchain for your device from <https://remarkablewiki.com/devel/toolchain> (e.g. `codex-x86_64-cortexa9hf-neon-rm10x-toolchain-3.1.15.sh`)
+* run that file to install the toolchain (e.g. `sudo sh codex-x86_64-cortexa9hf-neon-rm10x-toolchain-3.1.15.sh`)
+* source the printed environment file (e.g. `source /opt/codex/rm11x/3.1.15/environment-setup-cortexa7hf-neon-remarkable-linux-gnueabi`)
+* compile `main.c` using the `CC` environment variable (e.g. `$CC -O2 main.c`)
+    * if there is an error like `no such file or directory`, copy the command and execute it directly instead of using `$CC`, e.g. `arm-remarkable-linux-gnueabi-gcc  -mfpu=neon -mfloat-abi=hard -mcpu=cortex-a7 --sysroot=/opt/codex/rm11x/3.1.15/sysroots/cortexa7hf-neon-remarkable-linux-gnueabi -O2 main.c`)
+
 # TODO:
 - [ ] RM1 support (testers needed)
 - [ ] Left handed and landscape support for actions