feat: Add proper docs about our application tree

Author: rafaeelaudibert

MIGRATION.md

@@ -24,9 +24,22 @@ PostHog is consistently upgrading our internal data representation so that's bet
 
 ### Posthog.Application
 
-This library now depends on `Cachex`, and includes a supervision tree. To use features that require in-memory caching, you need to add `Posthog.Application` to your own supervision tree:
+This library now depends on `Cachex`, and includes a supervision tree. There are 2 options:
 
-#### Add to Your Supervision Tree
+1. If you have a simple application without a `YourApp.Application` application, then you can simply add `:posthog` to your `mix.exs` `application` definition
+
+```elixir
+def application do
+    [
+      extra_applications: [
+        # ... your existing applications ...
+        :posthog
+      ],
+    ]
+  end
+```
+
+2. Or, if you're already using an Application, you can add add `Posthog.Application` to your own supervision tree:
 
 ```elixir
 # lib/my_app/application.ex

README.md

@@ -40,6 +40,25 @@ def application do
   ]
 ```
 
+or if you already have a `YourApp.Application` application, you can also add `Posthog.Application` under your supervision tree:
+
+```elixir
+# lib/my_app/application.ex
+defmodule MyApp.Application do
+  use Application
+
+  def start(_type, _args) do
+    children = [
+      # Your other children...
+      {Posthog.Application, []}
+    ]
+
+    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end
+```
+
 ### Application Customization
 
 This library includes a `Posthog.Application` because we bundle `Cachex` to allow you to track inside PostHog your FF usage.