Update README with API versioning documentation

README.markdown

@@ -41,6 +41,17 @@ The OpenPhoto Python class hierarchy mirrors the [OpenPhoto API](http://theopenp
 * client.photos.list() -> /photos/list.json
 * photos[0].update() -> /photo/<id>/update.json
 
+<a name="api_versioning"></a>
+### API Versioning
+
+It may be useful to lock your application to a particular version of the OpenPhoto API.
+This ensures that future API updates won't cause unexpected breakages.
+
+To do this, add the optional ```api_version``` parameter when creating the client object:
+
+    from openphoto import OpenPhoto
+    client = OpenPhoto(host, consumerKey, consumerSecret, token, tokenSecret, api_version=2)
+
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 <a name="cli"></a>