Version Control With Subversion Source

are sweet, especially in PDF! So, kick things off with a couple of
index entries, just as a demonstration of what needs doin'.

* src/en/book/index.xml
New.

* src/en/book/book.xml
Hook in the new index.xml file.

* src/en/book/ch-preface.xml
Add a few indexterms, just as a demonstration.