Not all installations have build tools installed, and libsodium requires
calling out to pkg-config so add it to the build instructions on Debian
so that the build doesn't fail.
pkgconf would also work here, but most people just install pkg-config so I figured I'd put that one in the docs... I'm not sure the best way to say either/or on the apt install line.
Not all installations have build tools installed, and libsodium requires calling out to pkg-config so add it to the build instructions on Debian so that the build doesn't fail.