Add README.md for compiling C++ GUI

c++/README.md

+# README for C++ GUI
+
+## Compilation
+
+```sh
+mkdir build
+cd build
+cmake ..
+make
+./PersefoneGUI
+```
+
+You will need the necessary Qt packages installed.  On Ubuntu 22.04,
+these are (Note: this information is somewhat approximate, some
+packages could be missing, and some might not be necessary, sorry):
+
+### Prerequisites for building from source
+
+```sh
+sudo apt install build-essential cmake
+sudo apt install qt6-base-dev qt6-declarative-dev
+sudo apt install \
+    qml6-module-qtqml-workerscript \
+    qml6-module-qtquick \
+    qml6-module-qtquick-controls \
+    qml6-module-qtquick-dialogs \
+    qml6-module-qtquick-layouts \
+    qml6-module-qtquick-templates \
+    qml6-module-qtquick-tooling \
+    qml6-module-qtquick-window \
+    qt6-image-formats-plugins
+```