Add a note about Qt6 installed from pip and libxcb errors

INSTALL.md

@@ -45,6 +45,26 @@ The binaries for Python, GetText (`msgfmt`), `fpcalc` and `discid.dll` have to b
 in the `%PATH%` on Windows.
 
 
+Note about Qt6 installed via pip:
+---------------------------------
+
+If you get errors related to `libxcb` on application startup, you may need to install extra librairies on your system.
+
+Typical error looks like:
+
+```
+qt.qpa.plugin: From 6.5.0, xcb-cursor0 or libxcb-cursor0 is needed to load the Qt xcb platform plugin.
+qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
+This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
+```
+
+In this case, you have to install extra packages system-wide, for example, in this case, on Ubuntu:
+
+```bash
+sudo apt install libxcb-cursor0
+```
+
+
 Installation using pip
 ----------------------