Add custom aggregations user story

Author: dcherian

docs/source/custom.md

@@ -4,6 +4,8 @@
 It also allows you to specify a custom Aggregation (again inspired by dask.dataframe),
 though this might not be fully functional at the moment. See `aggregations.py` for examples.
 
+See the ["Custom Aggregations"](user-stories/custom-aggregations.ipynb) user story for a more user-friendly example.
+
 ```python
     mean = Aggregation(
         # name used for dask tasks

docs/source/user-stories.md

@@ -5,4 +5,5 @@
    :maxdepth: 1
 
    user-stories/climatology.ipynb
+   user-stories/custom-aggregations.ipynb
 ```