add docs for incidence_matrix

Author: Robbybp

docs/make.jl

@@ -32,6 +32,7 @@ makedocs(
             "reference/identify_variables.md",
             "reference/incidence_graph.md",
             "reference/interface.md",
+            "reference/incidence_matrix.md",
         ],
     ],
 )

docs/src/reference/incidence_matrix.md

@@ -0,0 +1,9 @@
+# Incidence matrix
+
+```@meta
+CurrentModule = JuMPIn
+```
+
+```@docs
+incidence_matrix
+```