Updated `README` and Logo

[serkor1]

Hi @TrevorFrench,

I wanted to make a humble contribution to your package and remove the white background of the sticker, I couldn't reproduce your logo entirely, so I decided to give an example (You can see it in action here).

This is the code I used to generate it with transparent borders:

library(hexSticker)
library(ggplot2)

p <- ggplot(aes(x = mpg, y = wt), data = mtcars) + geom_point()
p <- p + theme_void() + theme_transparent()

sticker(
  p,
  package="hexSticker", 
  p_size=20, 
  s_x=1,
  s_y=.75,
  s_width=1.3, 
  s_height=1,
  filename="inst/figures/ggplot2.png"
)

Feel free to reject the PR, and adapt your own logo if you find it useful

[TrevorFrench]

Thank you @serkor1!