If you want to disable the newly-added but quite annoying feature called the system integrity protection on OS X El Capitan, please follow the procedures below. This security feature disables users--even with the admin privilege--to modify /System, /sbin, and /usr directories, except /usr/local sub-directory.
First, you will need to enter recovery mode. To do this, you can simply press and hold [command] and [R] keys during the boot. However, if you are like me, using a mac mini plugged with a windows keyboard, it will be very difficult to enter recovery mode. In that case, what I did was to continually repeat the cycles of pressing and releasing both [Alt] keys very fast until the boot option menu pops up, at which point I can simply use the arrow keys to select recovery mode.
Second, once you are in the recovery mode, simply run the terminal from the drop down menu. In the terminal, run
# csrutil disable && reboot
You can always enable the feature back by running
# csrutil enable && reboot