Update documentation to recommend installing wagtail_modeladmin when using Wagtail 5.1+

Author: katdom13

docs/source/index.rst

@@ -30,14 +30,27 @@ Installation
 Use your favorite Python installer to install it from PyPI::
 
    pip install wagtail-robots
+   pip install wagtail_modeladmin  # if Wagtail >= 5.1
 
 Or get the source from the application site at::
 
    http://github.com/adrian-turjak/wagtail-robots/
 
 Then follow these steps:
 
-1. Add ``'wagtail_modeladmin'`` (if the Wagtail version is 5.1 and above, otherwise ``'wagtail.contrib.modeladmin'``) and ``'robots'`` to your INSTALLED_APPS_ setting.
+1. Add ``robots`` and ``modeladmin`` to the
+    ``INSTALLED_APPS`` setting in your project settings:
+
+   .. code-block:: python
+        :caption: e.g. settings.py
+
+        INSTALLED_APPS = [
+            ...
+            'wagtail_modeladmin',          # if Wagtail >=5.1; Don't repeat if it's there already
+            'wagtail.contrib.modeladmin',  # if Wagtail <5.1;  Don't repeat if it's there already
+            'robots',
+        ]
+
 2. Run the ``migrate`` management command
 
 You may want to additionally setup the `Wagtail sitemap generator`_.