Add a note about missing toolchain

New users may not realise that they need a toolchain in addition to the Qt libraries. Users should be guided to install the toolchain as well. 

Proposal for new texts:

macOS

Qt 6.0.2 Prebuilt Components for macOS Clang 64-bit toolchain. The toolchain is not included in the installation.

Linux

Qt 6.0.2 Prebuilt Components for Linux gcc 64-bit toolchain. The toolchain is not included in the installation.

Windows

Qt 6.0.2 Prebuilt Components for MSVC XX 64-bit toolchain. The toolchain is not included in the installation.

Qt 6.0.2 Prebuilt Components for MinGW XX 64-bit toolchain. The toolchain is available in the Developer and Designer Tools folder. 

Android

Qt 6.0.2 Prebuilt Components for Android armv7, arm64-v8a, x86 and x86_64 toolchains. The toolchains are not included in the installation.

iOS

Qt 6.0.2 Prebuilt Components for iOS Clang 64-bit toolchain. The toolchain is not included in the installation.