UPDATE:: Update README.md

Author: Malcolm White

README.md

@@ -1,18 +1,18 @@
 # Welcome to the *pykonal* repository!
-This code implements a mixed- (first- and second-) order finite-difference solver for the 2D Eikonal equation in Cartesian coordinates using the Fast-Marching Method of Sethian et al. (1996).
+This code implements a mixed- (first- and second-) order finite-difference solver for the 3D Eikonal equation in Cartesian coordinates using the Fast-Marching Method of Sethian et al. (1996).
 
 ## Planned development
-1. Extend to 3D
-2. Implement in spherical coordinates
+1. Implement in spherical coordinates
 
-## Example results
+### Compute synthetic arrival times in 3 dimensions:
 ![Figure 1](figures/figure_1.png)
+### For multiple sources:
 ![Figure 2](figures/figure_2.png)
+### And trace raypaths through heterogeneous velocity fields:
+#### Velocity model
 ![Figure 3](figures/figure_3.png)
+#### Travel time field
 ![Figure 4](figures/figure_4.png)
-![Figure 5](figures/figure_5.png)
-![Figure 6](figures/figure_6.png)
-![Figure 7](figures/figure_7.png)
 
 ## References
 Sethian, J. A. (1996). A fast marching level set method for monotonically advancing fronts. Proceedings of the National Academy of Sciences, 93(4), 1591–1595. https://doi.org/10.1073/pnas.93.4.1591