README.md should mention support for both Qt5 and Qt6

[NielsMayer]

https://github.com/QUItCoding/qnanopainter/blame/d050c93111d269522c4d4cdc0ac9c5f227c043a1/README.md#L4

Also there should be mention that the Qt5 verssion requires use of 'qt5' branch, or users will be surprised by errors compiling and running. This might be particularly confusing for those with Linux distros packaging Qt5.15.3 as the default Qt library (e.g. Ubuntu 22.04LTS).