Provide link to Apple's documentation on Touch Icons

[kkirsche]

Provide link to Apple's documentation on Touch Icons

[paulirish]

This link would make more sense in the extend.md

Also while they originated in iOS, android has supported these icons for years so the Apple docs may not be the best resource here.

[arthurvr]

Provide link to Apple's documentation on Touch Icons

This link would make more sense in the extend.md

This stuff is already really good documented in there.

But as Paul said, we should probably extend that explanation to be not that Apple-specific. Also, I'm not in favor of linking up the Apple docs as some stuff there is wrong, and still doesn't seem to be fixed.

I'm going to close this PR for now, as it doesn't seem to be the ideal fix. I agree the docs on touch icons need an update, so dropping it into #1554.