Add section about the RESET animation in Introduction to the animation features (#5628)

Co-authored-by: Hugo Locurcio <[email protected]>

tutorials/animation/introduction.rst

@@ -373,3 +373,30 @@ click on the keyframe and use the keyframe settings in the inspector.
 .. |Play from beginning| image:: img/animation_play_from_beginning.png
 .. |Add Animation| image:: img/animation_add.png
 .. |Add track| image:: img/animation_add_track.png
+
+Using RESET tracks
+------------------
+
+You can set up a special *RESET* animation to contain the "default pose".
+This is used to ensure that the default pose is restored when you save
+the scene and open it again in the editor.
+
+For existing tracks, you can add an animation called "RESET" (case-sensitive),
+then add tracks for each property that you want to reset.
+The only keyframe should be at time 0, and give it the desired default value
+for each track.
+
+If AnimationPlayer's **Reset On Save** property is set to ``true``,
+the scene will be saved with the effects of the reset animation applied
+(as if it had been seeked to time ``0.0``).
+This only affects the saved file – the property tracks in the editor stay
+where they were.
+
+If you want to reset the tracks in the editor, select the AnimationPlayer node,
+open the **Animation** bottom panel then choose **Apply Reset** in the
+animation editor's **Animation** dropdown menu.
+
+When adding tracks on new animations, the editor will ask you to automatically
+create a RESET track when using the keyframe icon next to a property in the inspector.
+This does not apply on tracks created with Godot versions prior to 3.4,
+as the animation reset track feature was added in 3.4.