add Word Cloud to public and develop docs

doc/public/course_data_formats/word_cloud/word_cloud.rst

+**********************************************
+Xml format of "Word Cloud" module [xmodule]
+**********************************************
+
+.. module:: word_cloud
+
+Format description
+==================
+
+The main tag of Word Cloud module input is:
+
+.. code-block:: xml
+
+    <word_cloud> ... </word_cloud>
+
+The following attributes can be specified for this tag::
+
+    [display_name| AUTOGENERATE] – Display name of xmodule. When this attribute is not defined - display name autogenerate with some hash.
+    [num_inputs| 5] – Number of inputs.
+    [num_top_words| 250] – Number of max words, which will be displayed.
+
+.. note::
+
+   If you want to use the same word cloud (the same storage of words), you must use the same display_name value.
+
+Example
+=======
+
+Examples of word_cloud without all attributes (all attributes get by default)
+-----------------------------------------------------------------------------
+
+.. code-block:: xml
+
+    <word_cloud />
+
+Examples of poll with all attributes
+------------------------------------
+
+.. code-block:: xml
+
+    <word_cloud display_name="cloud" num_inputs="10" num_top_words="100" />
\ No newline at end of file

doc/public/index.rst

@@ -26,6 +26,7 @@ Specific Problem Types
    course_data_formats/graphical_slider_tool/graphical_slider_tool.rst
    course_data_formats/poll_module/poll_module.rst
    course_data_formats/conditional_module/conditional_module.rst
+   course_data_formats/word_cloud/word_cloud.rst
    course_data_formats/custom_response.rst