Custom commands with Click v7.x turn underscores to dashes

Starting Click v7.0 subcommands that are named by the function automatically have underscores replaced with dash (see the 4th item from the bottom for the changelog of Click v7.0 at http://click.palletsprojects.com/en/7.x/changelog/).
This caused the Custom Commands example in the documentation not working with Click v7.x, so I have added a warning to Click v7.x users.

docs/cli.rst

@@ -270,6 +270,16 @@ This example adds the command ``create_user`` that takes the argument
 
     $ flask create_user admin
 
+.. warning:: `Click`_ v7.0 has introduced a change (see `v7.0 changelogs`_):
+    "Subcommands that are named by the function now automatically have the
+    underscore replaced with a dash. If you register a function named
+    ``my_command`` it becomes ``my-command`` in the command line interface."
+    So starting with `Click`_ v7.0  the command is: ::
+
+    $ flask create-user admin
+
+.. _v7.0 changelogs: http://click.palletsprojects.com/en/7.x/changelog/
+
 This example adds the same command, but as ``user create``, a command in a
 group. This is useful if you want to organize multiple related commands. ::