add div to surround Appear tag children

[ElreyB]

Description

Appear can only have one child to behave correctly which caused issue #597. CodePane has a child of its own. When it is inside an Appear tag it changes the slide index...

Slide with index of 0
Slide with index of 1(Slide with an `CodePane` inside an `Appear` tag)
Slide with index of 2

to

Slide with index of 0
Slide with index of 1
Slide with index of 0
Slide with index of 1
Slide with index of 2

Updated Readme docs to reflect this.

Fixes # 597

Type of Change

Please delete options that are not relevant.

• Update README (non-breaking change which fixes an issue)

How Has This Been Tested?

Test with a simple app

Checklist: (Feel free to delete this section upon completion)

• My code follows the style guidelines of this project (I have run yarn prettier-fix && yarn lint)
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules

[ElreyB]

@ebrillhart I updated my origin comment to reflect the changes I made to the Readme and also add a link for the Anim tag.

[ebrillhart]

this looks awesome, @ElreyB. thanks for figuring this out!!