add to README tutorial something on Headline

[eukreign]

Update the README with some instructions on how to use the Headline class and present matched excerpts from the result set....

https://github.com/damoti/django-tsvector-field/blob/master/tsvector_field/query.py#L5

from django.db.models.expressions import F
from django.contrib.postgres.search import SearchRank, SearchQuery
import tsvector_field

query = SearchQuery(self.form['text'].value())
qs = (
  qs.filter(text__search=query)
    .annotate(
      match_title=tsvector_field.Headline(F('text__title'), query),
      match_text=tsvector_field.Headline(F('text__text'), query)
    )
    .annotate(rank=SearchRank(F('text__search'), query))
    .order_by('-rank')
)

[3lixy]

Many thanks for how to use Headline. I almost had it right but this ticket got it working for me.