add width, height and sytle options README.md

[eins]

@ellisonleao thanks for creating glow.nvim
I really like how I can visualize a .md file really fast.

Is there any option for border ? the README.md said only shadow
I chose none and it worked :P

I also didn't find info about the max width or max height
I ended up setting up :

  border = "none",
  width_ratio = 99,
  height_ratio = 99,

Are there other options ?
Can we add that options to the README.md ?

Thanks