Re-write the Explainer

[m-alkalbani]

I am keen to re-write the explainer to better reflect the state and overall goal of the API. This will mostly aim to address some of the comments raised in #52 , mainly remove spec-like parts, cover purpose and rationale for the API and (maybe) some primer JS examples on getting started and using extensions rather than IDL definitions.

I want to follow W3C's Explainer Guidance as much as possible, and to make a separate PR for each of the following stages:

• Introduction, User-Facing Problem, Goals and Non-Goals.
• Proposed Approach: basic testing pattern overview, primer JS examples on getting started which can include connecting a fake device, simulating tracking loss/recovery, simulating input.
• Extension test hooks (overview).
• Extension test hooks (primer JS examples): include some examples for two or more extensions (refer to the spec for the rest of the extensions and future ones to be made).
• Considered alternatives (I might need some help and input on this as I wasn't around when the API was created).
• Stakeholder Feedback / Opposition (not sure if relevant for this).
• References & acknowledgements (more of a generic section if needed).

Any thoughts on this and would you be happy for me to make a PR for each of the above? @alcooper91 @Manishearth

[Manishearth]

Sure, that sounds fine

[alcooper91]

SGTM