Add complex example (#82)

* Add complex example

* Update changelog

Author: jorgensd

Changelog.md

@@ -4,7 +4,7 @@
 - Various minor DOLFINx API updates. `dolfinx.mesh.compute_boundary_facets` -> `dolfinx.mesh.exterior_facet_indices` with slightly different functionality. Use `dolfinx.mesh.MeshTagsMetaClass.find` instead of `mt.indices[mt.values==value]`.
 - Various numpy updates, use `np.full_like`.
 - Change all notebooks to use [jupytext](https://jupytext.readthedocs.io/en/latest/install.html) to automatically sync `.ipynb` with `.py` files.
-- Add dummy complex mode test
+- Add example of how to use `DOLFINx` in complex mode, see [chapter1/complex_mode].
 
 ## 0.4.1
 - No changes