Add "What is Qt Creator" to README.md

The description of Qt Creator as a product on qt.io is neither easy to find nor does it tell much about Qt Creator. In additions, the amount of information regarding Qt Creator does not exactly grow with each site update of qt.io.

The README.md of Qt Creator is mainly concerned with how to build the IDE from source and what the copyrights are. The description of what this repository contains is reduced to 1 sentence: Qt Creator is a cross-platform IDE for development with the Qt framework.

We can present our IDE better than that. Having a 2-3 screen pages long description in that README.md of what Qt Creator is and can do, with screenshots, would make sense. The advantages would be that the product information is maintained and contained in the product's own source tree, that it is versioned and that it automatically appears nicely formatted on popular pages such as GitHub/Gitlab.