title: Realm: Field Map inside another Map as RealmObject
body: <p>I need to parse json incoming from external system.
  Here json example:</p>
  
  <pre><code>{
      "address": "My address",
      "contacts": "My contacts",
      "exchangeRatesMap": {
          "CHF": {
              "field1": "7.103",
              "field2": "11.003"
  
          },
          "EUR": {
              "field1": "90.123",
              "field2": "1.003"
          },
          "USD": {
              "field1": "0.103",
              "field2": "20.123"
  
          }
      },
      "name": "Name of organization",
      "id": 1
  }
  </code></pre>
  
  <p>As result I create Java POJO (not Realm) in my Android application:</p>
  
  <pre><code>public class ExchangeRate {
      private int id;
          private String name;
          private String address;
          private String contacts;
          private Map&lt;String, Map&lt;String, String&gt;&gt; exchangeRatesMap = new HashMap&lt;String, Map&lt;String, String&gt;&gt;();
  
      }
  </code></pre>
  
  <p>And its work fine. OK.</p>
  
  <p>And now I need to migrate Java POJO to Realm Object.</p>
  
  <p>Question:
  How object <strong>ExchangeRates</strong> will be look as <strong>RealmObject</strong>?</p>