Tools: [skip ci] extend doc string of FreeCAD thumbnailer

src/Tools/freecad-thumbnailer

@@ -3,22 +3,32 @@
 
 Installation:
 - This executable file should be on the PATH so it can be found
-	"$ sudo cp freecad-thumbnailer /usr/share/bin"
-- the application/x-extension-fcstd MIME type should be registered
-	Check that a corresponding /usr/share/mime/packages/freecad.xml file exists
-  	Make sure the MIME database is up to date
-	"$ sudo update-mime-database /usr/share/mime"
-- Register this thumbnailer
-	Adding a file /usr/share/thumbnailers/FreeCAD.thumbnailer with the following content:
+    "$ sudo cp freecad-thumbnailer /usr/bin"
+  and must have execution rights
+    "$ sudo chmod +x /usr/bin/freecad-thumbnailer"
 
-	[Thumbnailer Entry]
-	TryExec=freecad-thumbnailer
-	Exec=freecad-thumbnailer -s %s %i %o
-	MimeType=application/x-extension-fcstd;
+- If a FreeCAD project file doesn't include a thumbnail the file freecad.png is used.
+  Thus, the file src/Gui/Icons/freecad-icon-48.png must be installed.
+    "$ sudo cp freecad-icon-48.png /usr/share/icons/hicolor/48x48/apps/freecad.png"
+
+- The application/x-extension-fcstd MIME type should be registered
+    Check that a corresponding /usr/share/mime/packages/freecad.xml file exists
+    Make sure the MIME database is up to date
+    "$ sudo update-mime-database /usr/share/mime"
+
+- Register this thumbnailer
+    Adding a file /usr/share/thumbnailers/FreeCAD.thumbnailer with the following content:
+
+    [Thumbnailer Entry]
+    TryExec=freecad-thumbnailer
+    Exec=freecad-thumbnailer -s %s %i %o
+    MimeType=application/x-extension-fcstd;
+
+HINT: Make sure that the symlink /usr/bin/python exists and points to the Python executable
 
 NOTE: To make sure FreeCAD saves thumbnail information:
 
-	Edit -> Preferences... -> Document -> Save thumbnail into project when saving document
+    Edit -> Preferences... -> Document -> Save thumbnail into project when saving document
 """
 import sys
 import zipfile