Add documentation for whiteboard

Issue #159
Signed-off-by: Michal Čihař <michal@cihar.com>

docs/admin/index.rst

@@ -18,4 +18,5 @@ Administrators guide
     config
     sample
     management
+    whiteboard
     advertisement

docs/admin/whiteboard.rst

+Whiteboard messages
+===================
+
+You can use whiteboard messages to give some information to your translators.
+The message can be site-wide or targeted on translation component or language.
+
+This can be useful for various things from announcing purpose of the website to
+specifying targets for translations.
+
+The whiteboard can currently be specified only in the admin interface:
+
+.. image:: ../images/whiteboard.png
+

weblate/html/admin/trans/change_form.html

@@ -7,8 +7,12 @@
 {% if opts.model_name == 'advertisement' %}
 {% doc_url 'admin/advertisement' as url %}
 {% else %}
+{% if opts.model_name == 'whiteboardmessage' %}
+{% doc_url 'admin/whiteboard' as url %}
+{% else %}
 {% doc_url 'admin/projects' opts.model_name as url %}
 {% endif %}
+{% endif %}
 <p>{% blocktrans %}Required fields are marked as bold, you can find more information in the <a href="{{ url }}">documentation</a>.{% endblocktrans %}</p>
 {% else %}
 <p class="errornote">{% blocktrans %}Do not change this object here, please use Weblate interface instead.{% endblocktrans %}</p>