Configuration entities are now available via the REST module use the GET method. This will allow clients to know for instance the labels of configuration entities such as Vocabularies and Content Types.
Currently since configuration entities don't have separate view permissions the clients will need to be logged in with administration permissions for the configuration entity types that need to be retrieve. For example to retrieve a vocabulary the client will need the 'Administer vocabularies and terms' permission.
Since most configuration entities do not have canonical links they will be available at the path 'entity/[ENTITY_TYPE]/[ENTITY_ID]'.
For instance to retrieve the tags vocabulary you can use:
curl --user admin:admin --request GET "http://drupal.d8/entity/taxonomy_vocabulary/tags?_format=json"
Configuration entities that do have canonical links, contact forms are the only example in core, will be available at their canonical link and not the path pattern above.
For instance to retrieve the Feedback contact form you can use:
curl --user admin:admin --request GET "http://drupal.d8/contact/feedback?_format=json"