Introduce templates directory with framer templates

Proposal

As we sometimes encounter problems/features which are quite difficult to visualise in static design mockups, we should make it as easy as possible to begin creating a prototype.

Framer is one of our tools which makes it possible to create both flow- and detail-prototypes.

This merge request adds a templates directory to the root of this repository including a framer templates directory.

The framer starting-prototype is documented in detail within the prototype, but I have added general documentation to the readme and contribution documentation.

The framer prototype includes the framer-material-kit for constraints and more, plus it has flow prototype functionality by default. It works off a sketch file, which is included.

Let me know if there are any thoughts, otherwise, let's get this merged 😉

/cc @cperessini @dimitrieh @hazelyang @pedroms @sarrahvesselov @sarahod @tauriedavis