docs: document that the built server auto-loads .env from cwd

- testing section now states dotenv behavior for the local build and
  that env variables take precedence over .env values
- verified empirically: .env with 2 vars -> 'injected env (2)' and
  server starts on stdio instead of aborting

README.md

@@ -232,6 +232,11 @@ node build/index.js --version
 # Run it manually (it then waits, silently, for MCP protocol frames on stdio):
 WIKIJS_URL=http://localhost:3000 WIKIJS_API_KEY=your-key node build/index.js
 
+# The built server auto-loads .env from the current working directory (dotenv),
+# so from the repo root this is sufficient: node build/index.js — provided
+# you created .env via the Setup steps above. Note: env variables take
+# precedence over .env values.
+
 # Interactive verify the tools without a full client setup (browser UI, default http://localhost:6274):
 npx @modelcontextprotocol/inspector node build/index.js      # pnpm: pnpm dlx @modelcontextprotocol/inspector node build/index.js
 ```