Add base documentation of Dispatcher (not ready, not fully documented)

docs/dispatcher/dispatcher.md

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+# Dispatcher
+Imports:
+
+- Recommended: `#!python3 from aiogram import Dispatcher`
+- Real location: `#!python3 from aiogram.dispatcher.dispatcher import Dispatcher`
+
+In code Dispatcher can be used directly for routing updates or attach another routers into dispatcher.
+
+!!! warning
+
+    Dispatcher is root [Router](router.md) and can not be included into another routers.
+
+
+Here is only listed base information about Dispatcher. All about writing handlers, filters and etc. you can found in next pages:
+
+- [Router](router.md).
+- [Observer](observer.md).
+
+## Simple usage
+Example:
+```python3
+dp = Dispatcher()
+
+@dp.message_handler()
+async def message_handler(message: types.Message) -> None:
+    await SendMessage(chat_id=message.from_user.id, text=message.text)
+```
+
+
+## Including routers
+Example:
+```python3
+dp = Dispatcher()
+router1 = Router()
+dp.include_router(router1)
+```
+
+
+## Handling updates
+All updates can be propagated to the dispatcher by `feed_update` method:
+
+```
+bot = Bot(...)
+dp = Dispathcher()
+
+...
+
+async for result in dp.feed_update(bot=bot, update=incoming_update):
+    print(result)
+```
+
+**Method specification**:
+
+| Argument | Type | Description |
+| --- | --- | --- |
+| `bot` | `Bot` | Bot instance related with current update object |
+| `update` | `Update` | Update object |
+| `**kwargs` | `Any` | Context related data. Will be propagated to handlers, filters and middlewares  |
+
+### Polling
+...
+
+### Webhook
+...