Add total video length to coordinates

src/2_coordinates.md

@@ -3,6 +3,10 @@ title: Coordinates
 ---
 # Coordinate systems
 
+The lecture on coordinate systems consists of 3 parts, each with their own video.
+
+Total video length: 35 minutes and 13 seconbds
+
 ## Introduction to coordinate systems: Cartesian and polar
 
 <iframe width="100%" height=315 src="https://www.youtube-nocookie.com/embed/CPMrsQlNxS8" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>