README optimized for dark mode

[rich-ayr]

Currently the Buckaroo logo as well as the How Buckaroo Works image aren't optimized for dark mode in the README. A possible solution for this would be to include an svg instead of a png and to add something like this:

<style>
  path {
    fill: #000;
  }
  @media (prefers-color-scheme: dark) {
    path {
      fill: #fff;
    }
  }
</style>

See https://www.reddit.com/r/github/comments/knb6c6/comment/gizm69r/?utm_source=share&utm_medium=web2x&context=3