docs: add Validation section to README with setup and configure instructions

Author: Bharatram-altimate-ai

README.md

@@ -157,6 +157,50 @@ Full docs at **[altimate.ai](https://altimate.ai)**.
 - [Agent Modes](https://altimate.ai/data-engineering/agent-modes/)
 - [Configuration](https://altimate.ai/configure/model-providers/)
 
+## Validation
+
+The `/validate` skill lets you audit past AI agent sessions against a set of quality criteria — checking whether the agent's reasoning, tool calls, and final response were correct, grounded, and complete. It pulls conversation traces from the backend, runs them through an evaluation pipeline, and reports per-criterion pass/fail results with details.
+
+You can validate:
+- **A single trace**: `/validate <trace_id>`
+- **All traces in a session**: `/validate --session-id <session_id>`
+- **A date range for a user**: `/validate --from <datetime> --to <datetime> --user-id <user_id>`
+
+### Setup
+
+**1. Register your API key**
+
+```bash
+altimate-code validate configure --api-key <your-key>
+```
+
+This registers your key with the Altimate validation server and saves it to `~/.altimate-code/settings.json`. The `/validate` skill reads credentials from that file when running evaluations.
+
+**2. That's it** — the skill files are installed automatically the next time you start `altimate-code`.
+
+To verify the installation:
+
+```bash
+altimate-code validate status
+```
+
+To install manually without restarting:
+
+```bash
+altimate-code validate install
+```
+
+### What happens if you skip configuration
+
+If you run `/validate` without configuring an API key first, the validation script will exit immediately with:
+
+```
+ERROR: Altimate credentials not found.
+Run: altimate validate configure --api-key <key>
+```
+
+No traces will be validated and nothing will be written. You must run `altimate-code validate configure` at least once before using the skill.
+
 ## Data Collection
 
 Altimate Code logs conversation turns (prompt, tool calls, and assistant response) to improve validation quality and agent behavior. Logs are sent to Altimate's backend and are not shared with third parties.