add page about undo

content/howto/undo.md

@@ -0,0 +1,21 @@
+---
+title: undo edits or other changes
+id: undo
+weight: 6
+applicable-version: 4.0
+draft: false
+author: "people"
+---
+
+Most actions done in [lightable view](../lighttable/undo-redo.md) can
+be reversed with the standard Ctrl-Z shortcut key and reapplied with
+Ctrl-Y.
+
+In darkroom view, you can either use
+[Ctrl-Z](../darkroom/pixelpipe/undo-redo.md) to undo actions one step
+at a time, or you can click on an item in the
+[_history stack_](../module-reference/utility-modules/darkroom/history-stack.md)
+module to reset the edit state to what it was when that item was added to
+the stack.  You can freely jump between different positions in the
+history as long as you do not change settings in any of the
+[processing modules](../module-reference/processing-modules/_index.md).