doc: add link

README.md

@@ -4,6 +4,8 @@ This repository contains the complete documentation related to
 [**Rotare**][rotare-repo], a feature-rich and open-source implementation of the
 Blade Element Momentum Theory in MATLAB.
 
+![https://img.shields.io/badge/Documentation-PDF-yellow?style=for-the-badge](https://rotare.gitlabpages.uliege.be/documentation/rotare-doc.pdf)
+
 ## IMPORTANT NOTE
 **Rotare is still currently in development. As long as it is not in v1.0.0, the
 documentation may not accurately represent the state of the code.**