Add diffstruc library to package index (#603)

Added diffstruc library for automatic differentiation.

Author: nedtaylor

data/package_index.yml

@@ -1200,6 +1200,13 @@
   tags: mpi cuda fft parallel distributed-memory
   license: GPL-3.0
 
+- name: diffstruc
+  github: nedtaylor/diffstruc
+  description: A library providing a derived type that enables forward and reverse mode automatic differentiation.
+  categories: numerical
+  tags: automatic-differentiation adjoint algorithmic derivative AD autodiff jacobian hessian
+  license: MIT
+
 # --- Scientific codes ---
 
 - name: astro-fortran