Improve readme for long metavars.

Add explicit mention of choices, add example and screenshots.

See #33

README.md

@@ -131,14 +131,17 @@ click.rich_click.GROUP_ARGUMENTS_OPTIONS = True
 
 > See [`examples/06_arguments.py`](examples/06_arguments.py) for an example.
 
-### Metavars
+### Metavars and option choices
 
 Metavars are click's way of showing expected input types.
 For example, if you have an option that must be an integer, the metavar is `INTEGER`.
 If you have a choice, the metavar is a list of the possible values.
 
 By default, rich-click shows metavars in their own column.
-However, with some tools this column can be quite wide and result in a lot of white space.
+However, if you have a long list of choices, this column can be quite wide and result in a lot of white space:
+
+![Default metavar display](https://raw.githubusercontent.com/ewels/rich-click/main/docs/images/metavars_default.png)
+
 It may look better to show metavars appended to the help text, instead of in their own column.
 For this, use the following:
 
@@ -147,6 +150,10 @@ click.rich_click.SHOW_METAVARS_COLUMN = False
 click.rich_click.APPEND_METAVARS_HELP = True
 ```
 
+![Appended metavar](https://raw.githubusercontent.com/ewels/rich-click/main/docs/images/metavars_appended.png)
+
+> See [`examples/08_metavars.py`](examples/08_metavars.py) for an example.
+
 ### Error messages
 
 By default, rich-click gives some nice formatting to error messages:

examples/08_metavars.py

@@ -0,0 +1,68 @@
+import rich_click as click
+
+click.rich_click.SHOW_METAVARS_COLUMN = False
+click.rich_click.APPEND_METAVARS_HELP = True
+
+
+@click.command()
+@click.option(
+    "--debug/--no-debug",
+    "-d/-n",
+    default=False,
+    help="""Enable debug mode.""",
+)
+@click.option(
+    "--number",
+    "-n",
+    type=click.Choice(
+        [
+            "one",
+            "two",
+            "three",
+            "four",
+            "five",
+            "six",
+            "seven",
+            "eight",
+            "nine",
+            "ten",
+            "eleven",
+            "twelve",
+            "thirteen",
+            "fourteen",
+            "fifteen",
+            "sixteen",
+            "seventeen",
+            "eighteen",
+            "nineteen",
+            "twenty",
+            "twenty-one",
+            "twenty-two",
+            "twenty-three",
+            "twenty-four",
+            "twenty-five",
+            "twenty-six",
+            "twenty-seven",
+            "twenty-eight",
+            "twenty-nine",
+            "thirty",
+        ]
+    ),
+    show_default=True,
+    help="This click choice has loads of options.",
+)
+def cli(debug, number):
+    """
+    My amazing tool does all the things.
+
+    This is a minimal example based on documentation
+    from the 'click' package.
+
+    You can try using --help at the top level and also for
+    specific group subcommands.
+    """
+    print(f"Debug mode is {'on' if debug else 'off'}")
+
+
+if __name__ == "__main__":
+    cli()