Add explainer document

[marcoscaceres]

A short explainer document to accompany the spec would be useful. A great example is @slightlyoff's one for Service Workers:

https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md

[marcoscaceres]

Elsewhere, @slightlyoff wrote:

The argument for why these operations need to be synchronous isn't clearly
presented (this looks and smells like I/O w/o a justified blocking
behavior, something we generally don't want). I was hoping the
authors would outline their thinking in a document that isn't
just spec + examples.

[marcoscaceres]

Should move the "this is today" examples in the spec to the explainer.

[AshleyScirra]

Added explainer.md.