Added a section on Dependency Injection technology in documentation (#1253)

* Added a section on Dependency Injection technology in documentation

* Added changelog

* Edited comments & titles

docs/dispatcher/dependency_injection.rst

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+####################
+Dependency injection
+####################
+
+Dependency injection is a programming technique that makes a class independent of its dependencies. It achieves that by decoupling the usage of an object from its creation. This helps you to follow `SOLID's <https://en.wikipedia.org/wiki/SOLID>`_ dependency inversion and single responsibility principles.
+
+
+How it works in aiogram
+=======================
+For each update :class:`Dispatcher` passes handling context data. Filters and middleware can also make changes to the context.
+
+To access contextual data you should specify corresponding keyword parameter in handler or filter. For example, to get :class:`FSMContext` we do it like that:
+
+.. code-block:: python
+    @router.message(ProfileCompletion.add_photo, F.photo)
+    async def add_photo(
+        message: types.Message, bot: Bot, state: FSMContext
+    ) -> Any:
+        ... # do something with photo
+
+
+Injecting own dependencies
+==========================
+
+Aiogram provides several ways to complement / modify contextual data.
+
+The first and easiest way is to simply specify the named arguments in :class:`Dispatcher` initialization, polling start methods or :class:`SimpleRequestHandler` initialization if you use webhooks.
+
+.. code-block:: python
+    async def main() -> None:
+        dp = Dispatcher(..., foo=42)
+        return await dp.start_polling(
+            bot, allowed_updates=dp.resolve_used_update_types(), bar="Bazz"
+        )
+Analogy for webhook:
+
+.. code-block:: python
+    async def main() -> None:
+        dp = Dispatcher(..., foo=42)
+        handler = SimpleRequestHandler(dispatcher=dp, bot=bot, bar="Bazz")
+        ... # starting webhook
+:class:`Dispatcher`'s workflow data also can be supplemented by setting values as in a dictionary:
+
+.. code-block:: python
+    dp = Dispatcher(...)
+    dp["eggs"] = Spam()
+The middlewares updates the context quite often.
+You can read more about them on this page:
+
+- `Middlewares <middlewares.html>`__
+
+The last way is to return a dictionary from the filter:
+
+.. literalinclude:: ../../../examples/context_addition_from_filter.py
+
+...or using MagicFilter with :code:`as_()` method. (`Read more <filters/magic_filters.html#get-filter-result-as-handler-argument>`__)