Update docs to include music app in INSTALLED_APPS

The current documentation was updated to inform users that they should include the "music" application in the INSTALLED_APPS list within Django's `settings.py` file. As this is a necessary step while setting up the application, it's crucial to have this information included in our documentation.

Author: lipemorais

docs/index.md

@@ -102,6 +102,22 @@ This will create the app structure for us. Something similar to this below:
     └── views.py
 ```
 
+Don't forget to add the app music to your INSTALLED_APPS in your `settings.py`
+```python
+# settings.py
+...
+INSTALLED_APPS = [
+    "django.contrib.admin",
+    "django.contrib.auth",
+    "django.contrib.contenttypes",
+    "django.contrib.sessions",
+    "django.contrib.messages",
+    "django.contrib.staticfiles",
+    "rest_framework",
+    "music",
+]
+...
+```
 ## Django Models
 
 Now the next step is create the models we are going to use in our API to represent the domain models.