docs: explain assignRelationship behavior

[elpete]

Issue review

Issue #150 asks Quick to document assignRelationship(), especially its ability to seed relationship data after creating records and avoid an unnecessary query.

I rate this 9/10 for Quick. The method is useful but easy to misunderstand: it controls the in-memory loaded relationship cache, not database association or persistence. Clear documentation prevents both extra queries and incorrect assumptions about foreign keys or pivot records. The only drawback is adding API detail to the repository README; the linked historical quick-docs repository has version branches but no next, so this PR keeps the requested target and the documentation beside the maintained API.

Implementation

• Added a README section with a public creation-and-seeding example.
• Documented singular versus collection values, replacement semantics, query avoidance, clearing, and lazy reloading.
• Explicitly documented that assignment does not save entities, synchronize keys, attach pivot rows, or validate relationship types.
• Expanded the method Javadoc so generated/source API documentation carries the same contract.
• Added public-API coverage proving an assigned relationship performs no query and clearRelationship() allows one normal lazy-load query afterward.

Validation

• Focused relationship-loading suite: 8 passed, 0 failed, 0 errors.
• Full suite: 496 passed, 0 failed, 0 errors, 3 skipped.
• box run-script format
• git diff --check

Closes #150

[elpete]

Moved to the canonical Quick documentation repository as ortus-docs/quick-docs#65.