README: Clarify Prerequisites

[luzpaz]

Will the editor require installing many KDE libs ? Please explain which it will install for users who are hesitant.

[Alonso-JAMM]

The main requirement for the editor is the package "KTextEditor" which in turn depends on a bunch of KDE libraries; however, all these libraries are provided by many Linux distributions. For example, in Arch Linux, installing the package "KTextEditor" would install all the required KDE libraries, or for debian I am pretty sure that the package "libkf5texteditor-dev" would install the libraries. In OpenSUSE the package should be "ktexteditor-devel," though I think for that it is needed to have the KDE repositories. For distributions that don't provide the development libraries for KTextEditor it may be needed to build them from source. KDE has several guides and tools for building from source the libraries:

1. https://kdesrc-build.kde.org/
2. https://community.kde.org/Guidelines_and_HOWTOs/Build_from_source/Install_the_dependencies
3. https://kate-editor.org/build-it/

But in order to build the python bindings for the editor the following packages are also needed:

1. Shiboken2
2. Pyside2
3. Qt5

These libraries are required for building FreeCAD so the wiki could be followed to get these dependencies.

However, I think the cmake files also need some work done in order to make them more reliable. For example, I hard-coded the include path for PySide2 (I just used "/usr/include/PySide2" which may not work on different systems).