Add target usages scenarios and more clarification in the beginning of the doc

[Chang Yan]

The term "Game engine / framework" is too big and ambiguous.

Right now we are at a point where our "game engine" is only limited for certain scenarios, for example, light-weight canvas game. It doesn't support WebGL for advanced rendering techniques. And the performance benchmark is quite unclear.

So I think it quite necessary that we clarify the use scenarios as specific as possible at the very beginning, rather than just say "it's a canvas game engine". I got this feedbacks from a Chinese programmer forum, v2ex.com.