Add example for SolrDocument + Mapstruct #68

It would be nice to have an example that shows how to use SolrDocument with Mapstruct.

I was trying to use MapStruct with SolrDocument and couldn't find a comprehensive example that could guide me . I was facing below issue during compiling:
Can't generate mapping method from iterable type to non-iterable type.
Reason is SolrDocument implements Iterable interface and Target DTOs doesn't .
So i came up with below solution to deal with this.
Source Class
SolrDocument
Here SolrDocument internally has Map<String,Object> _fields;

Destination Class:
`public class ItemDTO {

private String id;
private String displayName;
private String availability;
private String price;

//getter
//setter
}
}`

Error : Can't generate mapping method from iterable type to non-iterable type.

Solution:
Created SolrResponseWrapper around SolrDocument like below:
`public class SolrResponseWrapper {

private SolrDocument document;

public SolrResponseWrapper(SolrDocument document) {
this.document = document;
}

public String get(String key) {
return this.document.get(key).toString();
}
}`

Now Mapper is defined below for SolResponseWrapper to ItemDTO.

`@Mapper
public interface ItemMapper {
ItemMapper INSTANCE = Mappers.getMapper(ItemMapper);

@mappings({
@mapping(expression = "java(record.get("product_id"))", target = "id"),
@mapping(expression = "java(record.get("display_name"))", target = "displayName"),
@mapping(expression = "java(record.get("availability"))", target = "availability"),
@mapping(expression = "java(record.get("price"))", target = "price")
})
ItemDTO convertToItemDTO(SolrResponseWrapper record);
}`

If you think this is a good addition, please let me know. I will raise PR to have it included as an example.