Flesh out READMEHEADdev

Change-Id: I58d5e82c0fd04764d1bfb9bd068810b3d1a3790c
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

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+# QtMacExtras
+
+**The QtMacExtras module has been deprecated and is no longer part of Qt.**
+
+The functionality of the module has been incorporated into other parts of Qt, or in some cases
+removed due to being obsolete or better suited as cross platform APIs.
+
+For more details, see [QTBUG-93629](https://bugreports.qt.io/browse/QTBUG-93629) in the Qt issue
+tracker.
+
+## Porting away from QtMacExtras
+
+To learn more about how to port from QtMacExtras to alternative APIs please visit the
+[Qt 6 porting guide](https://doc.qt.io/qt-6/portingguide.html).
+
+## Working on Qt 5
+
+To work on patches for the Qt 5 series, check out a local branch of the relevant Qt version, e.g.:
+
+```bash
+$ git checkout 5.15
+Branch '5.15' set up to track remote branch '5.15' from 'origin' by rebasing.
+Switched to a new branch '5.15'
+```
+
+## Going back in time
+
+To inspect the code in the `dev` branch prior to its removal, follow these steps:
+
+```bash
+$ git checkout -b my-dev 1af4a00d7
+Switched to a new branch 'my-dev'
+```