Recommend introduction (scope and goal) on each document

[benoitbryon]

When a reader browse the documentation, he wants to quickly know what a document talks about. So that he can avoid reading something which he doesn't need to.
It's particularly important when looking for something: the topic of the document tells the reader whether he is looking at the right document or not.

So: write an introduction on each document.
Introduction should tell:

• topic
• scope
• goal