fix: Update docs

Author: HazAT

docs/manual-setup.rst

@@ -121,3 +121,22 @@ For that you need to add::
 
 We recommend leaving ``logLevel: "debug"`` since we look for specific ``sentry.properties``
 files depending on your flavors name.
+
+Please make sure your ``MainApplication.java`` looks something like this::
+
+.. sourcecode:: java
+    ...
+    import io.sentry.RNSentryPackage;
+    ...
+
+    public class MainApplication extends Application implements ReactApplication {
+
+        ....
+        @Override
+        protected List<ReactPackage> getPackages() {
+        return Arrays.<ReactPackage>asList(
+                new MainReactPackage(),
+                new RNSentryPackage(MainApplication.this)
+        );
+        }
+        ...