Recommend an introduction before H2

[benoitbryon]

See https://github.com/benoitbryon/documentation-best-practices/issues/3

In RST documents, if you use H2 just after H1 like this...

##########
Some title
##########


********
Subtitle
********

Here some text.

... then it means that there is no introduction text.

Should be:

##########
Some title
##########

This document illustrates usage of introduction text.


********
Subtitle
********

Here some text.