Add image of icon editor to the readme.md

[jimkring]

it would be nice if the readme file included an image of the icon editor, so that people can see a visual of what’s in the repository.

I borrowed this from the LabVIEW wiki page for icon editor: https://labviewwiki.org/wiki/Icon_Editor

[JayKayAce]

Furthermore, adding a map that shows the overview of which classes map to which part in the editor would also be a nice addition to the documentation of the Icon Editor

[jimkring]

@JayKayAce yes, it would be great to have a developer guide with overview of which classes map to which part in the editor