[offline] Update documentation: required ACLs in API key [ci skip]

The offline mode requires additional ACLs: browse and get settings.

Author: Clément Le Provost

doc/offline-mode.md

@@ -27,8 +27,21 @@ Offline features are brought by Algolia's **Offline SDK**, which is actually com
 
 ## Setup
 
+### Prerequisites
+
 1. Obtain a **license key** from [Algolia](https://www.algolia.com/).
 
+2. Make sure you use an **API key** with the following ACLs:
+
+    - Search (`search`)
+    - Browse (`browse`)
+    - Get index settings (`settings`)
+
+    *This is required because the offline mode needs to replicate the online index's settings and uses browse requests when syncing.*
+
+
+### Steps
+
 2. In your Gradle script, use the `algoliasearch-offline-android` package in place of `algoliasearch-android`. Typically:
 
     ```groovy