Change / add file in "Read only file system"

I want to add new command to my android 4.4.2. to do that I need to add alias line in the /system/etc/mkshrc file (bashrc in regular linux), or make symlink in the system/bin or system/xbin. but when I try to do so it says that "[command] filed for mkshrc :read-only file system". it occurs olso for ln -s. ls -l /system/etc give me that I can, as root, edit this file, but I can't.

How can I solve this problem?

EDIT

I installed busybox on my phone. It uses symlinks in system/xbin so there is a way to do so.