Added two various size of the robot-framework-logo

[bkhouri]

Here are the steps I performed.

1. Download the robot-framework.png
2. Open the file
3. Resize from 400x400 to 48x48
4. Export as a PNG
5. Resize from 48x48 to 24x24
6. Export as a PNG
7. Repeat the above steps for robot-framework-white.png

I know this may not be the best way to create the image, but I need logo's of these size.

[pekkaklarck]

The main idea of this repository is to have good quality originals and let those needing different sizes create them based on them. That said, having commonly used sizes available out-of-the-box would have its benefits. I just added SVG logo (#2) for similar reason.

A problem is deciding what icon sizes to consider common. It seems the situation is somewhat a mess on Windows alone and on mobile things get even more complicated. Related to this, on Windows you actually need *.ico files.

Due to the above reasons, I'm not sure does it make sense to add few different logo sizes. It's unlikely that they alone are enough and it's easy to create variants that are actually needed in different contexts. If you @bkhouri (or others) believe they'd be useful, I'm fine adding them. In that case following things should be done:

1. Investigate what are the actually often needed sizes. I don't think we need other formats than PNG.

2. Because image transformation always loses quality, it would be better to create smaller icon from the original, not from the version that was already scaled down once.

3. These new icons should be mentioned in README. I guess the current "PNG for web" could be changed to something like "PNG for web (24x24, 48x48)" where "24x24" and "48x48" are links to the actual logos.