Tool to write or draw annotations in the scene view.
Select Annotations from the Custom Tool dropdown in the SceneView Toolbar. When in this mode, an overlay shows up to allow you to change options such as stroke width, stroke colour, and the color of the drawing plane. Scene view navigation (eg. Alt-LMB to orbit or Alt-RMB to zoom) is still available.
To place a stroke, hold shift; the drawing plane will be displayed, and LMB dragging will draw a stroke. Undo and Redo are supported. The Clear button on the overlay panel will clear all strokes.
Currently the stroke container has its hideFlags set to HideAndDontSave, so the strokes won't contaminate your scenes.
Possibly looking into how best to handle persistent annotations that can be checked into source control without contaminating the scene.