Add clarification to docs about the rationale behind SVG requirements

[jxnblk]

[mikedemarais]

Converting SVG shapes into <path> elements
Most graphics applications can convert shapes into SVG paths.

- Adobe Illustrator: use the Compound Path command
- Figma: TK
- Sketch: TK

what does TK mean? https://github.com/c8r/pixo#converting-svg-shapes-into-path-elements

[jxnblk]

https://en.wikipedia.org/wiki/To_come_(publishing)

This particular issue is more about why it's good to use path elements in SVG. #3 is for adding instructions for different graphics applications