Update README — WEBLATE_ADD_APPS, Routes, and Celery

[AuraMindNest]

Problem

The README was expanded in Week 20 to cover the QuickBook format, settings-override.py, and general architecture. Week 21 adds significant new surface area: the boost_weblate Django app must be registered via WEBLATE_ADD_APPS, URL routes must be included in the Weblate URL conf, and the add-or-update endpoint requires a running Celery worker. None of this is documented in the README yet, and operators will fail to set up the plugin without it.

Acceptance Criteria

• README documents adding "boost_weblate" to WEBLATE_ADD_APPS in settings-override.py
• README documents including the URL conf: path("boost/", include("boost_weblate.urls")) (or the chosen mount point)
• README documents the Celery requirement: the add-or-update endpoint requires a Celery worker; include the command to start it (e.g., celery -A weblate.utils.celery worker)
• README includes a minimal "Quick Start" section that gets an operator from zero to a working plugin in 5 steps or fewer
• The architecture section is updated to reflect the new modules (views.py, tasks.py, services.py) and their relationships
• Existing README content (QuickBook format, WEBLATE_FORMATS, migration scripts) is preserved and not regressed

Implementation Notes

Place the new content in a "Deployment" or "Configuration" section, separate from the existing "Development" section. The Celery documentation should note that Weblate's own Celery instance is typically reused (same broker, same result backend) rather than starting a separate one. Mention that WEBLATE_ADD_APPS is a Weblate-specific setting (not standard Django INSTALLED_APPS) and link to the Weblate docs for context.

References

• Related files: README.md, settings-override.py
• Depends on: Andrew's apps.py/urls.py item (for exact app name and URL patterns)