add example for attachment with default field.
add example for attachment with default field.

rnewson/couchdb-lucene,rnewson/couchdb-lucene,eHealthAfrica/couchdb-lucene,ehealthafrica-ci/couchdb-lucene,ehealthafrica-ci/couchdb-lucene,eHealthAfrica/couchdb-lucene,rnewson/couchdb-lucene,ehealthafrica-ci/couchdb-lucene,eHealthAfrica/couchdb-lucene