docs(readme): fix license link, align tests w/ CI, add error handling

[martinkersner]

Summary

README cleanup, branched from latest origin/main.

• Fix broken License link (license.md → LICENSE).
• Quickstart is now env-var-first (Datamaxi() reads DATAMAXI_API_KEY; verified in datamaxi/api.py).
• Add data-coverage hook to intro; add "Discovery helpers" note under API Reference.
• Trim Premium's ~40-param dump to common filters + link to docs for the full set.
• Add Error Handling section (exception table + try/except example) and TOC entry.
• Explain the requirements/ files in a table.
• Tests: offline command now pytest tests/ -k "not integration" -q to match CI (old test_api.py-only command undercounted; missed test_call.py / test_query_params.py).

Doc-only change; no code touched.

Tests

• uv run pytest tests/ -k "not integration" -q → 108 passed, 11 skipped, 119 deselected.
• No known failures.

Notes

• tox.ini removal (from the original review) was moot — it's not on origin/main, only untracked locally.
• Python 3.10+ claim verified accurate against pyproject.toml (requires-python = ">=3.10").