* add asynchronous design doc

* update distributed_training.png

* add distributed_training.graffle

* add async_distributed_training.png

* update design doc

* change name to distributed_training_overview

* follow comment

* change distributed_training_overview.md to README.md

* update sync_distributed_training.png

* follow comment

* fix typo

* Add channel design document

* Update channel send/recv state diagram

* Create go_op design doc

change the path of image code

* Create select_op design document

* Fix pre-commit issues

* Update select op as per varun's comments

repair link of "Executor"