added inital socket mode document

Author: stevengill

docs/_basic/socket_mode.md

@@ -0,0 +1,55 @@
+---
+title: Using Socket Mode
+lang: en
+slug: socket-mode
+order: 16
+---
+
+<div class="section-content">
+With the introduction of [Socket Mode](https://api.slack.com/socket-mode), Bolt for Python introduced support in version `1.2.0`. With Socket Mode, instead of creating a server with endpoints that Slack sends payloads too, the app will instead connect to Slack via a WebSocket connection and receive data from Slack over the socket connection. Make sure to enable Socket Mode in your app configuration settings. 
+
+To use the Socket Mode, add `SLACK_APP_TOKEN` as an environment variable. You can get your App Token in your app configuration settings under the **Basic Information** section. 
+</div>
+
+```python
+import os
+from slack_bolt import App
+from slack_bolt.adapter.socket_mode import SocketModeHandler
+
+# Install the Slack app and get xoxb- token in advance
+app = App(token=os.environ["SLACK_BOT_TOKEN"])
+
+if __name__ == "__main__":
+    # export SLACK_APP_TOKEN=xapp-***
+    # export SLACK_BOT_TOKEN=xoxb-***
+    SocketModeHandler(app, os.environ["SLACK_APP_TOKEN"]).start()
+```
+
+While we recommend using [the built-in Socket Mode adapter](https://github.com/slackapi/bolt-python/tree/main/slack_bolt/adapter/socket_mode/builtin), there are a few other 3rd party library based implementations. Here is the list of available adapters. 
+
+|PyPI Project|Bolt Adapter|
+|-|-|
+|[slack_sdk](https://pypi.org/project/slack-sdk/)|[slack_bolt.adapter.socket_mode](https://github.com/slackapi/bolt-python/tree/main/slack_bolt/adapter/socket_mode/builtin)|
+|[websocket_client](https://pypi.org/project/websocket_client/)|[slack_bolt.adapter.socket_mode.websocket_client](https://github.com/slackapi/bolt-python/tree/main/slack_bolt/adapter/socket_mode/websocket_client)|
+|[aiohttp](https://pypi.org/project/aiohttp/) (asyncio-based)|[slack_bolt.adapter.socket_mode.aiohttp](https://github.com/slackapi/bolt-python/tree/main/slack_bolt/adapter/socket_mode/aiohttp)|
+|[websockets](https://pypi.org/project/websockets/) (asyncio-based)|[slack_bolt.adapter.socket_mode.websockets](https://github.com/slackapi/bolt-python/tree/main/slack_bolt/adapter/socket_mode/websockets)|
+
+To use the asyncio-based adapters such as aiohttp, your app needs to be compatible with asyncio's async/await programming model. `AsyncSocketModeHandler` is available for running `AsyncApp` and its async middleware and listeners. 
+
+```python
+from slack_bolt.app.async_app import AsyncApp
+# The default is the aiohttp based implementation
+from slack_bolt.adapter.socket_mode.async_handler import AsyncSocketModeHandler
+
+app = AsyncApp(token=os.environ["SLACK_BOT_TOKEN"])
+
+async def main():
+    handler = AsyncSocketModeHandler(app, os.environ["SLACK_APP_TOKEN"])
+    await handler.start_async()
+
+if __name__ == "__main__":
+    import asyncio
+    asyncio.run(main())
+```
+
+To learn how to use `AsyncApp`, checkout the [Using Async](https://slack.dev/bolt-python/concepts#async) document and relevant [examples](https://github.com/slackapi/bolt-python/tree/main/examples).