Add digital twin definition

Context: https://awortmann.github.io/research/digital_twin_definitions/

Did I miss it or is 

> Edward Glaessgen and David Stargel. 2012. The digital twin paradigm for future NASA and US Air Force vehicles. In 53rd AIAA/ASME/ASCE/AHS/ASC structures, structural dynamics and materials conference 20th AIAA/ASME/AHS adaptive structures conference 14th AIAA. 1818.

Really missing?