Add privacy pass reference

[smhendrickson]

Tried to describe differences without re-explaining the world.

[tfpauly]

@smhendrickson I updated the branch to move to an official appendix. I also updated the text to align better with the current protocol. Please check!

[smhendrickson]

@tfpauly lgtm with minor changes above. Thanks!

[tfpauly]

@smhendrickson thanks! In your last commit, I only see whitespace changes.. did everything get pushed?

[smhendrickson]

@tfpauly should be fixed. There is probably a clean way to squash those

[tfpauly]

Great, looks good!