Version Control With Subversion Source

Git would be better than SVN") to the degree that I plan to do so.

* en/book/ch00-preface.xml
(svn.intro.righttool): Rework this section to make a distinction
between "choosing version control" and "choosing Subversion", and
then try to make an honest attempt to quickly explain how
Subversion differs from the DVCS tools and why one might (or might
not) choose it over such a tool.