I added some sample screenshots on the README for the pixel shader feature
Fixes#8960
## Detailed Description of the Pull Request / Additional comments
I created 4 screenshots using preview windows terminal, one for the default look, and three for the shaders mentioned in the readme.
For the first shader, `Invert.hlsl`, I put it in a table and placed a screenshot of the default next to it. I don't think it would be clear what the shader did without being able to compare it against the default.
For `Rasterbars.hlsl` I put a screenshot right after the code sample for it. I added a screenshot for `Retro.hlsl` just before the last sentence
I also created a folder named Screenshots in `terminal/samples/Pixelshaders` to hold all of the screenshots.