Use .DEFAULT_GOAL to assign default target

[TommyLin]

Useing .DEFAULT_GOAL can assign absolute default target no matter where this line located.

[yungyuc]

Cool. I didn't know this.

But the note is written in the way to show existing approaches and conventions. It is quite popular to use an "alias" to redirect to a later target. And it also demonstrates the use of .PHONY.

I'll see how to include .DEFAULT_GOAL to the note. I think we need additional texts. Further update is welcome.