added documentation on how to create a search

CRM/Dataprocessor/Form/Field.php

Civi/DataProcessor/Factory.php

Civi/DataProcessor/FilterHandler/AbstractFilterHandler.php

docs/how_to_create_search.md

docs/images/howto_create_search_1.png

docs/images/howto_create_search_10.png

docs/images/howto_create_search_11.png

docs/images/howto_create_search_12.png

docs/images/howto_create_search_2.png

docs/images/howto_create_search_3.png

docs/images/howto_create_search_4.png

docs/images/howto_create_search_5.png

docs/images/howto_create_search_6.png

docs/images/howto_create_search_7.png

docs/images/howto_create_search_8.png

docs/images/howto_create_search_9.png

images/screenshot.png