Update README.md

+ Fixed all supported markdown violations. + Updated repo banner Removed white bg from the image. i.e: It was too bright for people who use dark themes on their browsers; like me :) + Fixed all grammatical errors I could find. + Changed snippet highlighting to PowerShell. To do all this stuff I used [removebg](https://www.remove.bg/) + [markdownlint](https://open-vsx.org/extension/DavidAnson/vscode-markdownlint) and [znck.grammarly](https://open-vsx.org/vscode/item?itemName=znck.grammarly) vscode extensions. I hope this isn't too much :)