Improve setup message

[changkun]

If the clipboard package is used in a cloud server environment, it may still not be possible to use the package on Linux. This is because x11 requires a frame buffer for GUI features.

This message:

clipboard/clipboard_linux.go

Lines 45 to 47 in e75e871

	panic(`cannot use this package, failed to initialize x11 display, maybe try install:
	apt install -y libx11-dev

Suggests to install libx11-dev may not be helpful, change to the following might be helpful:

failed to initialize the x11 display, and the clipboard will not work properly. Install the following
dependency may help:
 
	apt install -y libx11-dev

If the clipboard package is in an environment without a frame buffer, such as a cloud server, it may also
necessary to install xvfb:

	apt install -y xvfb

and initialize a virtual frame buffer:

	Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
 	export DISPLAY=:99.0

If we don't write this in the package panic message, we may document this in the README.md

Issue discovered by @qcrao