Useful VTT keyboard shortcuts table

[chrissimpkins]

There are a number of good shortcut nuggets embedded in the setup documentation text. It might be useful to include a table of your most commonly used keyboard shortcuts somewhere in this section. If this is included under a linkable header it would serve as a good browser bookmark reference to these docs.

[mikedug]

Broke out the, most used VTT shortcuts, into a linkable header, after the VTT Design Space setup section

[chrissimpkins]

https://github.com/googlefonts/how-to-vtt#useful-keyboard-shortcuts

Looks great. Thanks Mike!