Add a documentation section to the README

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

README.md

@@ -25,6 +25,11 @@ Then, just install using `pip`:
 python3 -m pip install frequenz-sdk
 ```
 
+## Documentation
+
+For more information, please visit the [documentation
+website](https://frequenz-floss.github.io/frequenz-sdk-python/).
+
 ## Contributing
 
 If you want to know how to build this project and contribute to it, please