Fix: Add code block to improve contribution docs (#9969)

docs/contributing/developing.md

@@ -44,7 +44,13 @@ Compile the assets:
 npm run build
 ```
 
-Any Wagtail sites you start up in this virtualenv will now run against this development instance of Wagtail. We recommend using the [Wagtail Bakery demo site](https://github.com/wagtail/bakerydemo/) as a basis for developing Wagtail. Keep in mind that the setup steps for a Wagtail site may include installing a release version of Wagtail, which will override the development version you've just set up. In this case, you should install the site before running the `pip install -e` step, or re-run that step after the site is installed.
+Any Wagtail sites you start up in this virtualenv will now run against this development instance of Wagtail. We recommend using the [Wagtail Bakery demo site](https://github.com/wagtail/bakerydemo/) as a basis for developing Wagtail. Keep in mind that the setup steps for a Wagtail site may include installing a release version of Wagtail, which will override the development version you've just set up. In this case, to install the local Wagtail development instance in your virtualenv for your Wagtail site:
+
+```sh
+pip install -e path/to/wagtail"[testing, docs]" -U
+```
+
+Here, `path/to/wagtail`is the path to your local Wagtail copy.
 
 (development_on_windows)=