Make the README runnable from a cold start

The setup steps assumed you had already cloned the repo and had a
working Python. They now start from git clone, name the version
required, and create a virtual environment before installing.

Author: Ark2027

README.md

@@ -6,8 +6,21 @@ I wrote this after spending a week working out why a published figure was wrong.
 
 ## Try it
 
+Needs Python 3.11 or newer. Nothing else — no database, no API keys, no accounts.
+
 ```bash
+git clone https://github.com/Ark2027/impact-data-quality
+cd impact-data-quality
+
+python -m venv .venv
+source .venv/bin/activate      # Windows: .venv\Scripts\activate
+
 pip install -e .
+```
+
+Then build the fixtures and audit them:
+
+```bash
 python scripts/generate_fixtures.py
 python run_audit.py fixtures/submissions --config config/example.json
 ```