readme.md logo update
readme.md logo update

Using logo with transparent background for better visuals on dark mode.

Signed-off-by: Jonathan Woollett-Light <69be128bc4ab81e6d91bd099cafe962594c4eb63@gmail.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker