add dual contours

Author: sjkelly

README.md

@@ -9,6 +9,7 @@ Algorithms included:
 * [Marching Tetrahedra](https://en.wikipedia.org/wiki/Marching_tetrahedra)
 * [Marching Cubes](https://en.wikipedia.org/wiki/Marching_cubes)
 * [Naive Surface Nets](https://0fps.net/2012/07/12/smooth-voxel-terrain-part-2/)
+* [Dual Contours](https://en.wikipedia.org/wiki/Isosurface#Dual_Contouring)
 
 ## Interface
 
@@ -44,6 +45,7 @@ Three meshing algorithms exist:
 * `MarchingCubes()`
 * `MarchingTetrahedra()`
 * `NaiveSurfaceNets()`
+* `DualContours()`
 
 Each takes an optional `iso` and `eps` parameter, e.g. `MarchingCubes(0.0,1e-6)`.