website update

web/www/tutorial12/tutorial12.html

@@ -28,6 +28,12 @@ <h1> Perspective Projection </h1>
 
             <iframe width="560" height="315" src="https://www.youtube.com/embed/md3jFANT3UM" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
                 <section>
+				<br>
+						<h3> Important note! </h3>
+						<p>The matrix developed in this tutorial is for a left handed coordinate system where the camera is aligned with the positive Z axis. This is why it may look a bit odd
+						if you are used to right handed coordinate systems. The GLM library is right handed by default so if you compare a GLM perspective projection matrix to the
+						one below you will see that there are two minor differences between the two versions. Once you've completed this tutorial try to develop the right handed
+						version as homework :-)  </p>
                         <h3> Background </h3>
 
                         <p>We have finally reached the