Add documentation for new whiteboard

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

docs/admin/whiteboard.rst

@@ -11,3 +11,30 @@ The whiteboard can currently be specified only in the admin interface:
 
 .. image:: ../images/whiteboard.png
 
+The whiteboard messages are then shown based on specified context:
+
+No context specified
+    
+    Shown on dashboard (landing page).
+
+Project specified
+
+    Shown on project, all its components and translations.
+
+Component specified
+
+    Shown on component and all its translations.
+
+Language specified
+
+    Shown on language overview and all translations in this language.
+
+
+You can see how it looks on the language overview page:
+
+.. image:: ../images/whiteboard-language.png
+
+
+And on the project page:
+
+.. image:: ../images/whiteboard-project.png

docs/changes.rst

@@ -24,6 +24,7 @@ Released on ? 2015.
 * Added option to specify source language.
 * Improved support for XLIFF files.
 * Extended list of options for import_project.
+* Improved targeting for whiteboard messages.
 
 weblate 2.4
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