Docs: Document how to create migrations

# Last 5 commit messages
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# 30eead5 Remove custom user app from sandbox [Maik Hoepfel]
# ee3ee02 Drop create_migration.py helper [Maik Hoepfel]
# a77e78c Don't show country field if only one is available [David Winterbottom]
# b5790aa Handle products without a category in GA subission [David Winterbottom]
# 39e0932 Merge pull request #782 from tangentlabs/feature/add_eway_extension_to_docs [David Winterbottom]

docs/source/internals/contributing/development-environment.rst

 
 Fork the repo and run::
 
-    git clone git@github.com:<username>/django-oscar.git
-    cd django-oscar
-    mkvirtualenv oscar  # using virtualenvwrapper
-    make install
+    $ git clone git@github.com:<username>/django-oscar.git
+    $ cd django-oscar
+    $ mkvirtualenv oscar  # using virtualenvwrapper
+    $ make install
 
 The :doc:`sandbox </internals/sandbox>` site can be used to test our changes in
 a browser. It is easily created with ``make sandbox``.
+
+Creating migrations
+===================
+
+As the sandbox is a vanilla Oscar site, it is what we use to build migrations
+against::
+
+    $ make sandbox
+    $ sites/sandbox/manage.py schemamigration $YOURAPP --auto
     
 Writing LESS/CSS
 ================