Updated usage documentation

docs/usage.md

@@ -44,6 +44,43 @@ Similarly to obtain informations about a specific user you can make a request li
 
     response = mkm.market_place.user(user='SampleUser')
 
-Note that if you need to send data to MKM backend it has to be XML or it will return a 400 Bad Request.
+Data can be posted directly as an XML string or formatted in a way that the XMLSerializer can parse it to correctly generate an XML string.
 
-[1]: http://docs.python-requests.org/en/latest/api/?highlight=response#requests.Response
+For example this:
+
+      data = {
+        'action': 'add',
+        'article': [
+          {'idArticle': 666, 'amount': 2},
+          {'idArticle': 101, 'amount': 5}
+        ]
+      }
+
+serializes to:
+
+      """
+      <?xml version="1.0" encoding="utf-8"?>
+        <request>
+          <action>add</action>
+          <article>
+            <idArticle>666</idArticle>
+            <amount>2</amount>
+          </article>
+          <article>
+            <idArticle>101</idArticle>
+            <amount>5</amount>
+          </article>
+        </request>
+      """
+
+It's important that dictionaries are always inside a list or they won't be serialized correctly, so if you have only one article to serialize you format it like this:
+
+    data = {
+      'article': [
+        {'idArticle': 101, 'amount': 10}
+      ]
+    }
+
+This is the same format used by the MKM backend for their JSON responses.
+
+[1]: http://docs.python-requests.org/en/latest/api/?highlight=response#requests.Response