Ch 3: Add subject of rationale

[Jeff Johnson]

At the top of Chapter 3, three bullet points describe what is
required in the root filesystem. There follows a "Rationale"
section. However, that section does not provide a Rationale for
the preceding section; instead it implores the reader to keep
the root filesystem small. Prior to the Rationale section
there needs to be a statement for which the section could be
the Rationale; e.g., "On the other hand, the required content
of the root filesystem should be kept to a minimum." The
first sentence of the Rationale should be changed to suit.