README: Improve Instruction on how to use this Addon

[luzpaz]

Proposal to add a screenshot/s and some extra text content + tweeks

What I'm confused by is the Usage section. Would it be OK break this down a little more and elaborate?

[luzpaz]

Would it be possible to make

For the demonstration purposes demo.rcc (for legacy support DemoTheme.zip) icon theme was provided. Move the demo.rcc (for legacy support extract the DemoTheme.zip archive) to an appropriate location.

A little more user friendly? I still actually don't know why this step is mandatory. It seems without it the Addon doesn't work.

OS: Manjaro Linux (XFCE/xfce)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.18009 (Git) AppImage
Build type: Release
Branch: master
Hash: 0641ca0d1e020d18f23d2adb49959195266f04f7
Python version: 3.7.3
Qt version: 5.12.1
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/United States (en_US)

Using most up-to-date release of IconThemes

[triplus]

Documentation will come at the end of development. I don't plan to put much thought into it, until then. As for the demo icon theme pack. It's there for the demonstration purposes. If you want to use more complete icon pack, look and ask here:

https://forum.freecadweb.org/viewtopic.php?f=34&t=34687

If you want to discuss anything further, best to ask in the dedicated forum thread:

https://forum.freecadweb.org/viewtopic.php?f=22&t=17901