docs: added drf-hstore to third party fields

added a reference to django-rest-framework-hstore in docs/api-guide/fields.md
docs: added HStoreSerializer to third party serializers

added a reference to django-rest-framework-hstore in docs/api-guide/serializers.md
Merge pull request #1790 from nemesisdesign/patch-6

docs: added HStoreSerializer to third party serializers
Merge pull request #1789 from nemesisdesign/patch-5

docs: added drf-hstore to third party fields
Merge remote-tracking branch 'upstream/master' into fix-1719