Updates README.md to reflect modern SPM syntax

[SamuelSchepp]

I updated the instruction to integrate GRMustache into a Swift Package project.

I also removed following line, because I recommend to remove the example project, since SPM is now more common and the example is no longer necessary.

Check [groue/GRMustacheSPM](https://github.com/groue/GRMustacheSPM) for a sample Swift package that uses GRMustache.swift.