chore: update .gitignore with dev-helpers/examples (#6373)

* docs: update setting-up.md with local dev

.gitignore

@@ -11,6 +11,7 @@ selenium-debug.log
 chromedriver.log
 test/e2e/db.json
 docs/_book
+dev-helpers/examples
 
 # dist
 flavors/**/dist/*

docs/development/setting-up.md

@@ -30,6 +30,22 @@ Unsupported Node.js LTS that should still work:
 5. Wait a bit
 6. Open http://localhost:3200/
 
+### Using your own local api definition with local dev build
+
+You can specify a local file in `dev-helpers/index.html` by changing the `url` parameter. This local file MUST be located in the `dev-helpers` directory or a subdirectory. As a convenience and best practice, we recommend that you create a subdirectory, `dev-helpers/examples`, which is already specified in `.gitignore`.
+
+replace
+```
+url: "https://petstore.swagger.io/v2/swagger.json",
+```
+
+with
+```
+url: "./examples/your-local-api-definition.yaml",
+```
+
+Files in `dev-helpers` should NOT be committed to git. The exception is if you are fixing something in `index.html` or `oauth2-redirect.html`, or introducing a new support file.
+
 ## Bonus points
 
 - Swagger UI includes an ESLint rule definition. If you use a graphical editor, consider installing an ESLint plugin, which will point out syntax and style errors for you as you code.