Document step to create database

[CI skip]

Fixes nijel/weblate-docker#1
Signed-off-by: Michal Čihař <michal@cihar.com>

docs/admin/install.rst

@@ -807,7 +807,13 @@ Example for Mac OS, similar on Windows, Linux, ...
 
     docker-compose build
 
-6. Start weblate containers:
+6. Create Weblate database:
+
+.. code-block:: sh
+
+    docker-compose run migrate
+
+7. Start weblate containers:
 
 .. code-block:: sh