Added a reminder to update migrations in tutorial

Feel free to change my copy for this change since the copy in the tutorial says that we don't need to do any migration, but in my case, I had to create the migrations (to add highlight and owner), deleting the database was not enough, I don't know if I did anything wrong.

Note: This was using Django 1.7.1.
I think it'd need to be this?...

    rm tmp.db
    rm -r snippets/migrations
    python manage.py makemigrations snippets
    python manage.py migrate