Added another example for a custom panel

[SergeoLacruz]

Example with a button calling a function and explanations on how the code works.

[SchrodingersGat]

@SergeoLacruz thanks for the extra example, a few simple spelling / text issues :)

[SergeoLacruz]

Thanks for checking.

[SchrodingersGat]

Thanks for the contribution @SergeoLacruz :)

[eeintech]

@SergeoLacruz When adding new screenshots and copy-pasting the code, don't forget to update the with id="xxx" with something different for the id value each time as right now, all screenshot in panel.md are using with id="panels" and when you click on any of them it shows the first screenshot:

Click on this screenshot:

It shows:

[SchrodingersGat]

@eeintech good catch! @SergeoLacruz if you have time can you issue another quick PR to update?