Update the automated install guide to cover private macOS install

[cmcgee1024]

There is some additional information needed for fully private macOS swiftly
installations. Add this information to the automated installation guide.

[heckj]

This would be easier to parse if you laid this out directly:

Suggested change

	You could be installing swiftly on a shared user account for macOS. In that case you might want to extract the pkg in a private location where it cannot be affected by other processes running in the same user account.
	If you are installing swiftly on a shared user account for macOS, extract the `pkg` in a private location where it cannot be affected by other processes running in the same user account.