improve usage code

README.md

@@ -92,8 +92,19 @@ https://web.archive.org/web/20101010101708/http://www.google.com/
 ##### CDX API aka CDXServerAPI
 ```python
 >>> from waybackpy import WaybackMachineCDXServerAPI
->>> url = "https://github.com"
+>>> url = "https://pypi.org"
 >>> user_agent = "Mozilla/5.0 (Windows NT 5.1; rv:40.0) Gecko/20100101 Firefox/40.0"
+>>> cdx = WaybackMachineCDXServerAPI(url, user_agent, start_timestamp=2016, end_timestamp=2017)
+>>> for item in cdx.snapshots():
+...     print(item.archive_url)
+... 
+https://web.archive.org/web/20160110011047/http://pypi.org/
+https://web.archive.org/web/20160305104847/http://pypi.org/
+.
+. # URLS REDACTED FOR READABILITY
+.
+https://web.archive.org/web/20171127171549/https://pypi.org/
+https://web.archive.org/web/20171206002737/http://pypi.org:80/
 ```
 
 > Documentation at <https://github.com/akamhy/waybackpy/wiki/Python-package-docs>.