add readme

README.md

@@ -9,9 +9,9 @@
 ```python
 import wps
 wps.run_app()
-wps.open('test.xlsx')
-wps.add_inline_picture()
-wps.close()
+wb = wps.open('test.xlsx')
+wb.add_inline_picture()
+wb.close()
 wps.exit()
 ```