PO Translations chapter added to the FDP Primer

[Warren Block]

The PO Translations chapter has been added to the FreeBSD Documentation 
Project Primer:
https://www.freebsd.org/doc/en_US.ISO8859-1/books/fdp-primer/po-translations.html

Please provide feedback.  Can the examples be improved?  Are there 
sections that are not clear?

At present, there is no example of the steps to add a new language 
directory.  It might also benefit from a more detailed example of 
modifying a book Makefile for the translated version.

Comments are welcome.

Thanks!