Merge branch 'master' into document-group

# Conflicts:
#	src/zarr/core/group.py

Author: brokkoli71

docs/tutorial.rst

@@ -330,10 +330,10 @@ representation of the hierarchy, e.g.::
 
     >>> root.tree()
     /
-     └── foo
-         └── bar
-             ├── baz (10000, 10000) int32
-             └── quux (10000, 10000) int32
+    └── foo
+        └── bar
+            ├── baz (10000, 10000) int32
+            └── quux (10000, 10000) int32
 
 The :func:`zarr.convenience.open` function provides a convenient way to create or
 re-open a group stored in a directory on the file-system, with sub-groups stored in
@@ -424,6 +424,12 @@ Groups also have the :func:`zarr.hierarchy.Group.tree` method, e.g.::
          ├── bar (1000000,) int64
          └── baz (1000, 1000) float32
 
+
+.. note::
+
+   :func:`zarr.Group.tree` requires the optional `rich <https://rich.readthedocs.io/en/stable/>`_
+   dependency. It can be installed with the ``[tree]`` extra.
+
 If you're using Zarr within a Jupyter notebook (requires
 `ipytree <https://github.com/QuantStack/ipytree>`_), calling ``tree()`` will generate an
 interactive tree representation, see the `repr_tree.ipynb notebook