Added steps to fix Xcode 10 'Multiple commands produce…' build issue

[cheloveq]

Added a few lines to the Installation docs explaining how to solve build error "Multiple commands produce…" in Xcode 10.
Fixes #3608

[henrikra]

Maybe also add note that this happens on XCode 10?

[Fabian-Fynn]

Copy only when installing is not a fix.
Each time yarn is executed the checkbox has to be ticked again.

Isn't there a way to set this permanently?

[cheloveq]

@henrikra
I did mention that in the first sentence. Anywhere else I should put it?

@Fabian-Fynn
That is true and I added that as a note following the steps. Personally I am not aware if it's possible to set it permanently, this is a workaround.

[rajivshah3]

#4321 essentially does this, but permanently. The Copy Files phase is creating duplicate files

[yogevbd]

Fixed here:b5d300f, no need to go through does steps anymore

[rajivshah3]

@yogevbd you beat me to it 😄