Add a README section for xcb plugin problem

Linux users may encounter a problem of missing
xcb plugin while using Qt6 built from sources.
Missing plugin prevents QtCreator from starting
with default settings. The new section provides
solution gathered from Qt forum

Change-Id: I2917965c99fa6a05feac0d1506c7e60ff63796b1
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

README.md

@@ -107,6 +107,33 @@ get LLVM.
     cmake -DCMAKE_BUILD_TYPE=Debug -G Ninja "-DCMAKE_PREFIX_PATH=/path/to/Qt;/path/to/llvm" /path/to/qtcreator_sources
     cmake --build .
 
+#### Troubleshooting: libxcb plugin not found while using Qt libraries built locally from source
+
+Ensure all prerequisites for building Qt are installed:
+https://doc.qt.io/qt-6/linux.html
+https://doc.qt.io/qt-6/linux-requirements.html
+
+If they were installed before building Qt and xcb plugin is missing try reinstall them with
+
+```sh
+    sudo apt-get --reinstall <package_name>
+```
+
+Reset building configuration for Qt libraries at '/path/to/qt_sources'
+
+```sh
+    cmake --build . --target=clean
+```
+
+and remove CMakeCache.txt
+
+```sh
+    rm CMakeCache.txt
+```
+
+Try building Qt source again.
+
+
 ### Windows
 
 These instructions assume that Ninja is installed and in the `PATH`, Qt Creator