readme: small cleanup
readme: small cleanup

What was originally just a 3 != 5 turned into a small yak shave. Cleans
up the README a bit to describe all of the functionality that go-systemd
now provides.

faiq/go-systemd,jonboulle/go-systemd,mikkeloscar/go-systemd,brnv/go-systemd,coreos/go-systemd,mikkeloscar/go-systemd,glerchundi/go-systemd,jonboulle/go-systemd,coreos/go-systemd,brnv/go-systemd,endocode/go-systemd,Yhgenomics/go-systemd,endocode/go-systemd,faiq/go-systemd,jareksm/go-systemd,jareksm/go-systemd,glerchundi/go-systemd,Yhgenomics/go-systemd