Update readme (#134)

* Add mention about the book

* Update README.md

* Update README.md

Author: Enforcer

README.md

@@ -1,14 +1,17 @@
 # Implementing the Clean Architecture - Auctions
-Example app.
+Example project used in the book [Implementing the Clean Architecture](https://leanpub.com/implementing-the-clean-architecture)
 
-[![CircleCI](https://circleci.com/gh/Enforcer/clean-architecture.svg?style=svg)](https://app.circleci.com/pipelines/github/Enforcer/clean-architecture?branch=master)
+[![Implementing the Clean Architecture cover](docs/cover.png)](https://leanpub.com/implementing-the-clean-architecture)
 
-# Work with the repo
+## Build status
+[![CircleCI](https://circleci.com/gh/Enforcer/clean-architecture.svg?style=svg)](https://app.circleci.com/pipelines/github/Enforcer/clean-architecture?branch=master) [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
+
+## Working with the repo
 Pycharm - mark each package as Sources Root.
 ![Marking directories as source root in PyCharm](docs/marking_as_sources_root.png)
 Console - `make dev` to install each subpackage in editable mode.
 
-# Work with docker containers
+## Working with docker containers
 ```bash
 # Start everything
 docker-compose up --build