Added build instruction

README.md

@@ -10,3 +10,25 @@ Please change `DOOR_PIN` accordingly.
 
 You need to provide the environment variables
 `SPACEAPI_URL` and `API_KEY`.
+
+## Build
+
+[Install stable version of rust](https://rustup.rs/) and run a regular cargo build:
+
+```sh
+cargo build
+```
+
+### Cross compile
+
+Install arm toolchain and *libmusl* for static linked binaries:
+
+```sh
+apt-get install gcc-arm-linux-gnueabihf musl-dev musl-tools
+```
+
+Build the application for raspberry pi:
+
+```sh
+cargo build --target=armv7-unknown-linux-musleabihf
+```