Add Dlint to extras documentation (#1270)

Hi there!

I remember working together a bit on another tool of mine, cohesion: https://github.com/mschwager/cohesion/issues?q=is%3Aissue+author%3Asobolevn.

It seems Dlint was considered for addition in #683. I was hoping to at least include it in your extras list.

Let me know if you need any additional information.

docs/pages/usage/integrations/extras.rst

@@ -9,5 +9,6 @@ Things we highly recommend to improve your code quality:
 - `mypy <https://github.com/python/mypy>`_ runs type checks on your python code. Finds tons of issues. Makes your code better, improves you as a programmer. You must use, and tell your friends to use it too
 - `import-linter <https://import-linter.readthedocs.io>`_ allows you to define application layers and ensure you do not break that contract. Absolutely must have
 - `cohesion <https://github.com/mschwager/cohesion>`_ tool to measure code cohesion, works for most of the times. We recommend to use it as a reporting tool
+- `dlint <https://github.com/dlint-py/dlint>`_ tool for encouraging best coding practices and helping ensure Python code is secure
 - `vulture <https://github.com/jendrikseipp/vulture>`_ allows you to find unused code. Has some drawbacks, since there is too many magic in python code. But, it is still very useful tool for the refactoring
 - `bellybutton <https://github.com/hchasestevens/bellybutton>`_ allows to write linters for custom use-cases. For example, it allows to forbid calling certain (builtins or custom) functions on a per-project bases. No code required, all configuration is written in ``yaml``