misc/ios: add to README how to build ios executables

Updates #43371, #43343.

Change-Id: I19386269245f2c20345c6cac7560089b8223e9f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/280153
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: cherrymui

misc/ios/README

@@ -7,6 +7,13 @@ set to the clang wrapper that invokes clang for iOS. For example, this command r
 
 	GOOS=ios GOARCH=amd64 CGO_ENABLED=1 CC_FOR_TARGET=$(pwd)/../misc/ios/clangwrap.sh ./all.bash
 
+If CC_FOR_TARGET is not set when the toolchain is built (make.bash or all.bash), CC
+can be set at commond line. For example,
+
+	GOOS=ios GOARCH=amd64 CGO_ENABLED=1 CC=$(go env GOROOT)/misc/ios/clangwrap.sh go build
+
+Setting CC is not necessary if the toolchain is built with CC_FOR_TARGET set.
+
 To use the go tool to run individual programs and tests, put $GOROOT/bin into PATH to ensure
 the go_ios_$GOARCH_exec wrapper is found. For example, to run the archive/tar tests: