Improve the readme file

Adds a slightly more descriptive introduction to the project in the
README and a link to the Microgrid API website.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

README.md

@@ -6,9 +6,10 @@
 
 ## Introduction
 
-Microgrid API client for Python
-
-TODO(cookiecutter): Improve the README file
+This project provides a Python client for the [Frequenz Microgrid
+API](https://frequenz-floss.github.io/frequenz-api-microgrid/) that is more
+idiomatic to use in Python than the automatically-generated gRPC client
+provided by `protoc`.
 
 ## Supported Platforms