README: clarify that arithmetic expansions perform nested expansions lazily

[rob-myers]

Your parser permits lazy evaluation of the ternary operator - ? - : -, and gosh does so:

~$ gosh
$ (( 1 + 1 == 2 ? 1 : $( echo foo >&2; echo 1) )) 
$  # bash would output foo here

This is great, but perhaps it should be optional or noted somewhere?

Source: http://mywiki.wooledge.org/BashFAQ/031#Theory.

[mvdan]

This is a good point. I think we could cover this together with the confusion about a='b + c'; (($a)) not working like in Bash.