Citations of third-party works in appendix only.

As alluded to in 2ea19b71d4a917babb29024f06acabfe73309f40 's commit
message on 2014-12-17 19:52:15 -0500, keeping any information on a
part-by-part basis is difficult and error-prone, since there exists no
reliable way to auto-generate such information accurately.

Therefore, citations to third-party works, in addition to remaining
fully documented in the commit log as they always have been, are now
placed in specifically one location in the body of the text itself: a
single appendix specifically designed for that purpose.

In this manner, contributors have no house-keeping work regarding
citations. Contributors need only list third party works and links in
one place: third-party-citations.tex.

Documentation in CONTRIBUTING.md for making contributions of third-party
works is left as a TODO.