Explain how to debug the uninstaller.

README.md

@@ -168,7 +168,7 @@ Source code tips
   ``OutputBaseFilename=setup`` in your script, and copy the resulting setup-*.bin
   files to the source code directory. This way you can simulate an actual
   installation while running under the Delphi debugger.
-
+  
 - All of the forms in the Setup project, with the exception of Main.dfm, have
   Scaled set to False. This is because they dynamically scale themselves at
   run-time by calling a function named InitializeFont.
@@ -181,6 +181,14 @@ Source code tips
 - For compiler debugging purposes define ``STATICCOMPILER`` in CompForm.pas
   and ``STATICPREPROC`` in Compile.pas.
 
+- To debug the uninstaller first run Setup.exe to completion with the
+  ``/DETACHEDMSG`` command line parameter set. Afterwards copy uninst000.dat and
+  uninst000.msg as setup.dat and setup.msg to the Projects directory in your
+  issrc path. Then open the Setup project and set the command line parameters to
+  ``/UNINSTMODE "/SECONDPHASE=<your issrc path\Projects\Setup.exe"`` and start
+  debugging. Note: each time setup.dat and setup.msg will be deleted if you
+  allow the uninstaller to complete so make sure to keep copies.
+
 
 Precompiled executables and libraries
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