docs: document thread-safety of client (#75)

Author: HemangChothani

docs/index.rst

@@ -1,5 +1,13 @@
 .. include:: README.rst
 
+.. note::
+
+ Because the Datastore client uses the :mod:`grpcio` library by default
+ and uses third-party :mod:`requests` library if the GRPC is disabled,
+ clients are safe to share instances across threads. In multiprocessing
+ scenarios, the best practice is to create client instances *after*
+ :class:`multiprocessing.Pool` or :class:`multiprocessing.Process` invokes
+ :func:`os.fork`.
 
 API Reference
 -------------