Adds appendix with PSK specifications.

[markafoltz]

This addresses Issue #210: [Security] Describe encoding/decoding of PSK into numeric and QR codes.

This appendix describes two ways to turn a PSK, a short pairing code used with SPAKE2, into a visual representation. The first way is as a numeric string in groups of 3 or 4 digits. The other way is with a QR code.

The sample QR code was generated by the following library and was tested on a Pixel phone.

QR-Code-generator

Note, the ISO specification for QR codes does not appear to be in specref, so I will add it as a custom biblio entry.

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