add very basic example

Author: wassup05

example/intrinsics/CMakeLists.txt

@@ -1,2 +1,3 @@
 ADD_EXAMPLE(sum)
-ADD_EXAMPLE(dot_product)
+ADD_EXAMPLE(dot_product)
+ADD_EXAMPLE(matmul)

example/intrinsics/example_matmul.f90

@@ -0,0 +1,7 @@
+program example_matmul
+    use stdlib_intrinsics, only: stdlib_matmul
+    complex :: a(2,2)
+    a = reshape([(0, 0), (0, -1), (0, 1), (0, 0)], [2, 2]) ! pauli y-matrix
+
+    print *, stdlib_matmul(a, a, a, a, a) ! should be sigma_y
+end program example_matmul