More docs on async mode

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
 
+- Async API requests.
 - One more description for StaleChat error.
 
 # 0.8.0

README.md

@@ -17,7 +17,8 @@ Package contains:
 - Middleware and routes helpers for production env.
 - Poller with automatic source-reloader for development env.
 - Rake tasks to update webhook urls.
-- Async requests for Telegram and/or Botan API. Let the queue adapter handle errors!
+- __[Async mode](#async-mode)__ for Telegram and/or Botan API.
+  Let the queue adapter handle network errors!
 
 Here is sample [telegram_bot_app](https://github.com/telegram-bot-rb/telegram_bot_app)
 with session, keyboards and inline queries.
@@ -384,11 +385,24 @@ you can implement your own worker class to handle such requests. This allows:
 - Handle and retry network and other errors with queue adapter.
 - ???
 
+Instead of performing request instantly client serializes it, pushes to queue,
+and immediately return control back. The job is then fetched with a worker
+and real API request is performed. And this all is absolutely transparent for the app.
+
 To enable this mode add `async: true` to bot's and botan's config.
 For more information and custom configuration check out
 [docs](http://www.rubydoc.info/github/telegram-bot-rb/telegram-bot/master/Telegram/Bot/Async) or
 [source](https://github.com/telegram-bot-rb/telegram-bot/blob/master/lib/telegram/bot/async.rb).
 
+Be aware of some limitations:
+
+- Client will not return API response.
+- Sending files is not available in async mode [now],
+  because them can not be serialized.
+
+To disable async mode for the block of code use `bot.async(false) { bot.send_photo }`.
+Yes, it's threadsafe too.
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies.