Adds an externs.js
Adds an externs.js

These were kind of a pain to generate, so I thought you might want to provide them.  I kept a single copyright preamble in place, and put this in the root directory, let me know if you want me to change either of those.
kyroskoh/gl-matrix,toji/gl-matrix,WoodMath/gl-matrix,kkuunnddaannkk/gl-matrix,3D4Medical/gl-matrix,nickdesaulniers/gl-matrix,verma/gl-matrix,Drakesinger/gl-matrix,sajjadt/gl-matrix,verma/gl-matrix,3D4Medical/gl-matrix,nickdesaulniers/gl-matrix,WoodMath/gl-matrix,verma/gl-matrix,kkuunnddaannkk/gl-matrix,toji/gl-matrix,sajjadt/gl-matrix,kyroskoh/gl-matrix,3D4Medical/gl-matrix,Drakesinger/gl-matrix,kyroskoh/gl-matrix