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.