Update README.md with project documentation

Co-authored-by: kendaleiv <1012917+kendaleiv@users.noreply.github.com>

Author: Copilot

README.md

@@ -1,2 +1,32 @@
 # t-sql-at-time-zone
-T-SQL AT TIME ZONE tests
+
+Integration tests exercising the T-SQL [`AT TIME ZONE`](https://learn.microsoft.com/en-us/sql/t-sql/queries/at-time-zone-transact-sql) clause against SQL Server 2022 using [Testcontainers](https://dotnet.testcontainers.org/).
+
+## Prerequisites
+
+- [.NET 10 SDK](https://dotnet.microsoft.com/download) (or later)
+- [Docker](https://www.docker.com/)
+
+## Running Tests
+
+```bash
+dotnet test
+```
+
+A SQL Server 2022 container is started automatically via Testcontainers.
+
+## Test Scenarios
+
+| Scenario | Windows Time Zone | IANA Time Zone |
+|---|---|---|
+| `datetime` without offset — assigns offset | `Eastern Standard Time` | `America/New_York` |
+| `datetimeoffset` — converts to target zone | `Pacific Standard Time` | `America/Los_Angeles` |
+| Daylight Saving Time transitions | `Eastern Standard Time` | `America/New_York` |
+| Chaining `AT TIME ZONE` | EST → PST | `America/New_York` → `America/Los_Angeles` |
+| `datetime2` with high precision | `US Eastern Standard Time` | |
+| `smalldatetime` | `Eastern Standard Time` | |
+| UTC to multiple zones (same instant) | EST / PST / CET | `America/New_York` / `America/Los_Angeles` / `Europe/Berlin` |
+| Half-hour offset (India) | `India Standard Time` | `Asia/Kolkata` |
+| Windows and IANA produce same result | `Eastern Standard Time` | `America/New_York` |
+
+IANA time zone tests are skipped automatically when the SQL Server instance does not support IANA time zone names.