Update docs image

[divanvisagie]

Update the screenshot in the readme to reflect the published version.

[Violet-XiaoWeiHuang]

Hi Divan,
I would like to contribute to this issue.
I wonder you want to update the README image from https://github.com/divanvisagie/fretboard/blob/master/docs/images/promo.gif

[divanvisagie]

Yes! That's absolutely awesome, would really appreciate it.

For the current gif i used this neat little app. https://www.cockos.com/licecap/

The idea was to show the selected notes being 'Transposed' when switching tunings.

Feel free to make suggestions on the readme as well, it's looking a little sparse at the moment.

[Violet-XiaoWeiHuang]

I didn't play guitar.
Could you explain more about

to show the selected notes being 'Transposed' when switching tunings.

Thanks

[divanvisagie]

If you take a look at what I'm doing in the original gif it's sort of the same thing. Each string follows the same pattern of A, A#, B, C , C#, D, D# , E, F, F# , G , repeat.

When we tune the guitar, we simply cause each string to start on a different note, when we change to a "different tuning" that means the strings start on different notes.

This means that in fretboard , when we select notes and change the tuning , the selected notes should move up and down the fretboard according to how the tuning has changed.