title: Findbugs error - RV_RETURN_VALUE_IGNORED when using ImmutableSet.Builder
body: <p>Findbugs throws warning for the following piece of code. Seems like a simple of <code>Builder</code> class to add items and construct at the end. Whats the issue here ?</p>
  <pre><code>public static Set&lt;Entity&gt; convert(@NonNull final String id,
          @NonNull final Collection&lt;SomeEntity&gt; list) {
  
          Builder&lt;Entity&gt; builder = new ImmutableSet.Builder&lt;&gt;();
          final Entity entityOfType1 = createEntity(..);        
    //    Bug type RV_RETURN_VALUE_IGNORED 
          builder.add(entityOfType1);
  
  
          final Set&lt;Entity&gt; entitiesOfType2 = createEntity(..);
   //     Bug type RV_RETURN_VALUE_IGNORED 
          builder.addAll(entitiesOfType2);
  
          final Set&lt;Entity&gt; entitiesOfType3 = createEntity(..);
   //     Bug type RV_RETURN_VALUE_IGNORED 
          builder.addAll(entitiesOfType3);
  
          return builder.build();
      }
  
  </code></pre>