contributors.md

Thanks to those who contributed code and ideas, including:

@aldebaran1 (robustness)
@ryanpavlick (.rsphere, .rcurve, .latitude, .lox, .los, .dca, and more)
@cchuravy (Ellipsoid parameters)
@jprMesh  (more conversion functions)
@Fil (docs)
@SamuelMarks (docs)
@Yozh2 (numerous ellipsoids and tests)