Add Emulator Documentation in SYSTEMS.md

This adds a [SYSTEMS.md](https://github.com/RobLoach/EmulationStation/blob/systems/SYSTEMS.md) which outlines how to add support for many different systems into **EmulationStation**. The goal is to document usage of a number of platforms, along with as many emulators as possible which support it. This allows users to easily copy and paste the XML into their `es_systems.cfg` without having to worry too much.

* [Preview the current version](https://github.com/RobLoach/EmulationStation/blob/systems/SYSTEMS.md#readme) too see what it would be like once merged.
* More emulators and platforms are to be added - Git allows easy collaboration on this
* @ProfessorKaos64 has [a great `es_systems.cfg`](https://github.com/ProfessorKaos64/RetroRig-ES/blob/master/es-cfgs/es_systems.cfg) set up that we could merge in