docs(readme): document new copilot provider config example

- add example copilot table with model, max_tokens, and reasoning_effort
- update README setup snippet to show copilot alongside existing providers

Author: aweis89

README.md

@@ -185,6 +185,11 @@ require("ai_commit_msg").setup({
       },
     },
   },
+  copilot = {
+    model = "gpt-5-mini",
+    max_tokens = 10000,
+    reasoning_effort = "minimal",
+  },
 })
 ```