Modify README to cross-compile deb packages for ARM in Pi-targeted builds

Currently if executed on an x86_64 system, the README's instructions will generate a deb package which contains x86_64 binaries (or whatever the host architecture is of the machine which compiles the code). This is contrary to the users' expectation, which is that they would build a deb package which can be installed on a Raspberry Pi.

This commit modifies the README to add the appropriate commands and setup steps necessary to build deb packages for Raspberry Pi systems, the intended target of this software. The specific commands were taken from the GitLab CI configuration that builds the deb packages in CI.

README.md

@@ -34,6 +34,8 @@ To build the software, you will need to install [Rustup](https://rustup.rs/). Yo
 ```bash
 sudo apt install git protobuf-compiler
 cargo install cargo-deb
+rustup target add aarch64-unknown-linux-gnu # necessary to build for all RPi models 2B and newer
+rustup target add arm-unknown-linux-musleabihf # necessary to build for RPi zero, and RPi 1
 ```
 
 After these are installed, restart your terminal, or open a new one. Then run these commands:
@@ -42,7 +44,16 @@ After these are installed, restart your terminal, or open a new one. Then run th
 git clone https://gitlab.scd31.com/cats/igate
 cd igate
 git pull
-cargo deb
 ```
 
-The resulting deb package will placed in `target/debian/cats-igate_VERSION.deb`. The package can be installed with `sudo dpkg -i target/debian/cats-igate_VERSION.deb`, replacing `VERSION` with whatever version and architecture was built.
+To build for modern Raspberry Pis (everything newer than Model 2B):
+```bash
+cargo deb --target aarch64-unknown-linux-gnu
+```
+
+To build for the Rasberry Pi Zero or Pi 1:
+```bash
+cargo deb --target arm-unknown-linux-musleabihf
+```
+
+The resulting deb package will placed in `target/$TARGET_ARCH/debian/cats-igate_VERSION_arm64.deb`. The package can be installed with `sudo dpkg -i target/$TARGET_ARCH/debian/cats-igate_VERSION.deb`, replacing `TARGET_ARCH` with the architecture specified in `cargo deb`, and `VERSION` with whatever version and architecture was built.