Fix #1322: Add note in docs about None not being sent as data

In the case:

  payload = {'key1': 'value1', 'key2': 'value2', 'key3': None}
  r = requests.get("http://httpbin.org", params=payload)

the parameter `key3` will not be sent as a parameter in the URL.

Mention this in the documentation.

docs/user/quickstart.rst

@@ -71,6 +71,9 @@ You can see that the URL has been correctly encoded by printing the URL::
     >>> print r.url
     u'http://httpbin.org/get?key2=value2&key1=value1'
 
+Note that any dictionary key whose value is ``None`` will not be added to the
+URL's query string.
+
 
 Response Content
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