add a link for the PDDL

Replacing Markdown footnote 6 to the Public Domain Mark which was unnecessarily left in after its use in the text was removed.
replacing the link to the 8 Principles with Carl's page, since the opengovdata.org domain is currently not working (and the change to link to my book in bf9fd7dc4fbde324aa73d53e701b4adeb4d54e5f was not what we wanted)
add brief document versioning information (a history/changelog) to the bottom