chore: Move doc from README to Getting Started guide

Author: cheerfulstoic

guides/introduction/Getting Started.md

@@ -20,8 +20,8 @@ To use EctoWatch, you need to add it to your supervision tree and specify watche
 
 This will setup:
 
- * triggers in PostgreSQL during application startup
- * an Elixir process for each watcher which listens for notifications and broadcasts them via `Phoenix.PubSub`
+* triggers in PostgreSQL during application startup
+* an Elixir process for each watcher which listens for notifications and broadcasts them via `Phoenix.PubSub`
 
 Then any process (e.g. a GenServer, a LiveView, a Phoenix channel, etc...) can subscribe to messages like so:
 
@@ -43,6 +43,8 @@ You can also subscribe to individual records:
   EctoWatch.subscribe({User, :deleted}, user.id)
 ```
 
+**NOTE:** you can't subscribe to the `:inserted` event for specific objects because the primary key's value which you would use to subscribe doesn't exist until the insert happens.
+
 ... OR you can subscribe to records by an association column (but the given column must be in the `extra_columns` list for the watcher! See below for more info on the `extra_columns` option):
 
 ```elixir
@@ -73,4 +75,3 @@ Once subscribed, messages can be handled like so (LiveView example are given her
     {:noreply, socket}
   end
 ```
-