Add iOS build instructions. Bug: angleproject:40096751 Change-Id: I3168663374906afbdec5a4ad5398704dbcf3731d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5812088 Auto-Submit: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>