Docs: document every driver option and split contributor docs

[tas50]

What

Adds a complete configuration reference, a quick start, and a CONTRIBUTING.md.

Why

The clouds.yaml / OS_* documentation added recently is genuinely good — but it covers credentials only. The Usage section immediately below it said:

See https://kitchen.ci/docs/drivers/openstack/ for documentation.

So the other 24 default_config keys — image, flavor, networking, floating IPs, timeouts — plus 8 options read directly from config were documented nowhere in the repo.

README

• A quick start that leans on source openrc.sh, since that's the path most OpenStack users already have working
• A short Credentials section framing the three sources and their precedence, pointing into the existing clouds.yaml docs rather than duplicating them
• Full configuration reference grouped by concern: image/flavor, instance, networking, waiting/timeouts, API/endpoints
• A Block device mapping section, showing the hash shape
• Examples: floating IPs, multiple networks, cloud-init, slow images, per-platform images
• Cinc Workstation as the install route, cinc kitchen commands, and a "Using with Chef" section

Every option is now documented. That includes cloud_config, disable_ssl_validation, flavor_id, flavor_ref, image_id, image_ref, server_wait and user_data, which are read from config but never declared through default_config.

Mutually exclusive options

Three pairs raise ActionFailed if you set both halves, which wasn't written down anywhere:

• image_ref + image_id
• flavor_ref + flavor_id
• cloud_config + user_data

The tables now say so at the point you'd reach for them.

CONTRIBUTING.md

New file, absorbing the README's Development section. It notes that kitchen diagnose is the way to see the settings actually in effect — the driver merges clouds.yaml into its config during finalize_config!, so diagnose reflects the combined result of clouds.yaml, OS_*, and kitchen.yml. That's the fastest way to characterise a credential-precedence bug.

The maintenance note (OSU Open Source Lab) is carried across.

Docs only — no code changes.