Add screenshots to usage guide

docs/usage.rst

@@ -28,6 +28,8 @@ Languages
 Choose here which languages you prefer to translate. These will be offered to
 you on main page to have easier access to translations.
 
+.. image:: _static/your-translations.png
+
 Secondary languages
 +++++++++++++++++++
 
@@ -47,6 +49,8 @@ You can subscribe to various notificatiosn on :guilabel:`Subscriptions` tab.
 You will receive notifications for selected events on chosen projects for
 languages you have indicated for translation (see above).
 
+.. image:: _static/profile-subscriptions.png
+
 Projects structure
 ------------------
 
@@ -67,6 +71,8 @@ existing translation (even if it has been changed meanwhile).
 
     This ability might be limited by :ref:`privileges`.
 
+.. image:: _static/export-import.png
+
 Translation links
 -----------------
 
@@ -76,6 +82,8 @@ strings. Should no other checks fire, there will be still link to all
 translations. Alternatively you can use search field to find translation you
 need to fix.
 
+.. image:: _static/strings-to-check.png
+
 Translating
 -----------