zephyr-env.sh: Add a note to indicate the required version of zsh

For zsh users, to successfully source this environment set up script,
the version of zsh need to be 5.0.6 or above. Users can download the
newest zsh at http://zsh.sourceforge.net/Arc/source.html, and then
install it manually.

Change-Id: I1877b4030cf8688bce6b2483c302f957f4a5506f
Signed-off-by: d0u9 <d0u9.su@outlook.com>

zephyr-env.sh

@@ -19,6 +19,9 @@
 # to the name of the function/script when executing a shell function or
 # sourcing a script. POSIX_ARGZERO option, when it is set, exposes the
 # original value of $0 in spite of the current FUNCTION_ARGZERO setting.
+#
+# Note: The version of zsh need to be 5.0.6 or above. Any versions below
+# 5.0.6 maybe encoutner errors when sourcing this script.
 if [ -n "$ZSH_VERSION" ]; then
 	DIR="${(%):-%N}"
 	if [ $options[posixargzero] != "on" ]; then