Adding an example for calculating the bounding box

[incetarik]

Hi, first of all thank you for your work.

I tried this library to measure a string in a given font. To test, I also had my SVG editor open and I saw that the characters bounds are 100% correct. Yet, I could not find a way to calculate the overall text bounding box. I mean the box I have in the SVG editor for the text itself, not the characters.

Could you please add an example for calculating the expected string bounding box, if there is a way to calculate that properly?