Ability to add descending order indexes (#262)

* DescIndex(column) for descending index columns, refs #260
* Ability to add desc indexes using CLI, closes #260

docs/cli.rst

@@ -1033,6 +1033,14 @@ Use the ``--unique`` option to create a unique index.
 
 Use ``--if-not-exists`` to avoid attempting to create the index if one with that name already exists.
 
+To add an index on a column in descending order, prefix the column with a hyphen. Since this can be confused for a command-line option you need to construct that like this::
+
+    $ sqlite-utils create-index mydb.db mytable -- col1 -col2 col3
+
+This will create an index on that table on ``(col1, col2 desc, col3)``.
+
+If your column names are already prefixed with a hyphen you'll need to manually execute a ``CREATE INDEX`` SQL statement to add indexes to them rather than using this tool.
+
 .. _cli_fts:
 
 Configuring full-text search

docs/python-api.rst

@@ -1866,6 +1866,17 @@ By default the index will be named ``idx_{table-name}_{columns}`` - if you want
         index_name="good_dogs_by_age"
     )
 
+To create an index in descending order for a column, wrap the column name in ``db.DescIndex()`` like this:
+
+.. code-block:: python
+
+    from sqlite_utils.db import DescIndex
+
+    db["dogs"].create_index(
+        ["is_good_dog", DescIndex("age")],
+        index_name="good_dogs_by_age"
+    )
+
 You can create a unique index by passing ``unique=True``:
 
 .. code-block:: python