Update basic demo to include viewbox

[sdiemert]

Great library! One suggestion for the README file.

Please include examples that setup up the viewbox. I spent about an hour trying to figure out why the most basic examples wouldn't integrate with my existing svgdotjs code. Turns out I wasn't setting the viewbox and I wouldn't have known to do this unless I dug into the issues on this repo.

A basic example might be:

// enables panZoom
var canvas = SVG()
  .addTo('#id')
  .size(1000, 1000)
  .viewbox('0 0 500 500')
  .panZoom()

// zoom programatically
canvas.zoom(lvl, point)

I understand that this will setup a 1000x1000 canvas and create 500x500 viewbox.

If you don't do something like this then one gets console errors like:

Error: <svg> attribute viewBox: Expected number...

Thank you!

[Fuzzyma]

Feel free to go ahead and create a pr which adds that to the Readme :)

[liweicheng00]

Thank you! This is really helpful.

[rainyspring]

me too

[BillyPurvis]

Super helpful, thank you @sdiemert!