update doc

Author: skywind3000

README.md

@@ -86,6 +86,7 @@ available options:
   --lang=xxx      optional, output language
   --concise       optional, generate concise message if present
   --utf8          optional, output utf-8 encoded text if present
+  --url=xxx       optional, alternative openai request url
 ```
 
 It can be integrated into other editors. 

bin/gptcommit.py

@@ -304,6 +304,7 @@ def help():
     print('  --lang=xxx      optional, output language')
     print('  --concise       optional, generate concise message if present')
     print('  --utf8          optional, output utf-8 encoded text if present')
+    print('  --url=xxx       optional, alternative openai request url')
     print()
     return 0