docs: add database support matrix with tier labels to README #32
Description
Problem
The README lists 13 supported databases as if they are all equally production-ready. In reality, driver completeness varies significantly. Users installing data-diff for BigQuery or Oracle hit NotImplementedError walls with no warning.
Proposed Tier System
| Tier | Label | Criteria | Databases |
|---|---|---|---|
| 1 | Production | All methods implemented, dedicated tests, CI coverage | PostgreSQL, MySQL, DuckDB |
| 2 | Stable | All core methods, basic tests | Redshift, Presto, Vertica, Snowflake, Databricks |
| 3 | Limited | Missing methods (timezone, OFFSET), no dedicated tests | BigQuery, Oracle, ClickHouse, MsSQL |
| 4 | Experimental | Stub/minimal implementation | Trino |
Scope
Add a "Database Support" section to README.md with:
- Tier labels and what they mean
- Table showing each database, its tier, and known limitations
- Link to the new-database-driver-guide for contributors
Acceptance Criteria
- README contains a clear support matrix
- Each database has a tier label
- Known limitations are listed per database
- Users can self-select appropriate databases before investing setup time