Add another post action (show_action) to README (#573)

Add another post action (show_action) to README (#573)

* Add another post action (show_action) to README

This commits appends another post action (show_action) to README.

Signed-off-by: lotieye.choi <lotieye.choi@samsung.com>
* Add Running with post action section

This commit adds `Running with post action` section.

Signed-off-by: lotieye.choi <lotieye.choi@samsung.com>
* Remove a unused symbol

This commit remove a unused symbol.

Signed-off-by: lotieye.choi <lotieye.choi@samsung.com>
* Modify sentence to clarify meaning

This commit modifies a sentence to clarify meaning

Signed-off-by: lotieye.choi <lotieye.choi@samsung.com>