Created Custom ObjectMapper (markdown)

Custom-ObjectMapper.md

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+How to use custom ObjectMapper?
+
+1. Create new Class and implement Interface EntityMapper
+```
+public class CustomEntityMapper implements EntityMapper {
+
+    public CustomEntityMapper(){
+        //custom configuration/implementation (e.g. FasterXML/jackson)
+    }
+
+    @Override
+    public String mapToString(Object object) throws IOException {
+        //mapping Object to String
+        return null;
+    }
+
+    @Override
+    public <T> T mapToObject(String source, Class<T> clazz) throws IOException {
+        //mapping String to Object
+        return null;
+    }
+}
+```
+2. Define new bean and inject it into ElasticsearchTemplate
+```
+    <bean name="elasticsearchTemplate"
+          class="org.springframework.data.elasticsearch.core.ElasticsearchTemplate">
+        <constructor-arg name="client" ref="client"/>
+        <constructor-arg name="entityMapper" ref="entityMapper"/>
+    </bean>
+
+    <bean name="entityMapper" class="org.springframework.data.elasticsearch.core.CustomEntityMapper"/>
+```
+3. Done 
+
+Default implementation for EntityMapper used in project can be found here https://github.com/spring-projects/spring-data-elasticsearch/blob/master/src/main/java/org/springframework/data/elasticsearch/core/DefaultEntityMapper.java