add documentation

Closes #91 (closed).

adaptive/__init__.py

@@ -16,7 +16,7 @@ with suppress(ImportError):
     # Only available if 'scikit-optimize' is installed
     from .learner import SKOptLearner
 
-from .runner import Runner, BlockingRunner
+from .runner import Runner, AsyncRunner, BlockingRunner
 from . import version
 
 __version__ = version.version