Merge pull request #54 from richdouglasevans/docs/root-validation

Document how validation only applies underneath the server's base URL

README.md

@@ -159,6 +159,27 @@ app.use((err, req, res, next) => {
 });
 ```
 
+## The Base URL
+
+The validator will only validate requests — and (optionally) responses — that are under
+the server's [base URL](https://spec.openapis.org/oas/v3.0.0.html#serverVariableObject).
+
+This is useful for those times when the API and frontend are being served by the same
+application. ([More detail about the base URL](https://swagger.io/docs/specification/api-host-and-base-path/).)
+
+```yaml
+servers:
+  - url: https://api.example.com/v1
+``` 
+
+The validation applies to all paths defined under this base URL. Routes in your app
+that are _not_ under the base URL—such as pages—will not be validated.
+
+| URL                                  | Validated?                  |
+| :----------------------------------- | :-------------------------- |
+| `https://api.example.com/v1/users`   | :white_check_mark:          |
+| `https://api.example.com/index.html` | no; not under the base URL  |
+
 ## [Example Express API Server](https://github.com/cdimascio/express-openapi-validator-example) (clone it)
 
 A fully working example lives [here](https://github.com/cdimascio/express-openapi-validator-example)