Move the docs into docs/ and keep them out of the dist

[adrorocker]

The readme had grown to 599 lines, which is too long to be a landing page and
too short to be a manual. Split it up.

README keeps the intro, what the package is and is not, installation, one
example and the history. Everything else moves to docs/:

• getting started - entities, repositories, connecting
• reading and writing - saving, batches, transactions, server values
• queries and modelling - the query builder, and what each backend can run
• errors - what gets thrown and what it carries
• extending it - other backends, other constraints, your own query methods
• upgrading from 0.x (moved from UPGRADING.md, history preserved)

docs/ is export-ignored, so composer require now pulls down only:

LICENSE  README.md  composer.json  src/

README stays at the root on purpose - Packagist and GitHub read it for the
project page.

Every relative link in the readme and the six pages was checked to make sure it
still resolves. No code changed; 165 tests still pass.