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.