Add "usage" command demo

demo/plugins/usage.py

@@ -0,0 +1,14 @@
+import nonebot
+from nonebot import on_command, CommandSession
+
+
+@on_command('usage', aliases=['使用帮助', '帮助', '使用方法'])
+async def _(session: CommandSession):
+    plugins = list(filter(lambda p: p.name, nonebot.get_loaded_plugins()))
+    arg = session.current_arg_text.strip().lower()
+    if not arg:
+        session.finish(
+            '我现在支持的功能有：\n\n' + '\n'.join(p.name for p in plugins))
+    for p in plugins:
+        if p.name.lower() == arg:
+            await session.send(p.usage)