doc(android): document IMA ads insertion activation

API.md

@@ -164,6 +164,21 @@ protected List<ReactPackage> getPackages() {
     );
 }
 ```
+
+#### Enable custom feature in gradle file
+
+##### Enable client side ads insertion
+To enable client side ads insertion CSAI with google IMA SDK, you need to enable it in your gradle file.
+
+```gradle
+buildscript {
+  ext {
+    ...
+    RNVUseExoplayerIMA = true
+    ...
+  }
+}
+```
 </details>
 
 ### Windows installation
@@ -370,6 +385,9 @@ Example:
 adTagUrl="https://pubads.g.doubleclick.net/gampad/ads?iu=/21775744923/external/vmap_ad_samples&sz=640x480&cust_params=sample_ar%3Dpremidpostoptimizedpodbumper&ciu_szs=300x250&gdfp_req=1&ad_rule=1&output=vmap&unviewed_position_start=1&env=vp&impl=s&cmsid=496&vid=short_onecue&correlator="
 ```
 
+Note: On android, you need enable IMA SDK in gradle file, see: [enableclient side ads insertion](#enable-client-side-ads-insertion)
+
+
 Platforms: Android, iOS
 
 #### allowsExternalPlayback