[skip ci] Update README.md

README.md

-WOLF - Modular estimation framework for robotics based on factor graphs | IMU Plugin
-====================================================================================
+WOLF IMU plugin - Modular estimation framework for robotics based on factor graphs
+==================================================================================
 
 For installation guide and code documentation, please visit the [documentation website](http://mobile_robotics.pages.iri.upc-csic.es/wolf_projects/wolf_lib/wolf-doc-sphinx/).