Add explanation text to budget to have single expense per line

[hspeijer]

Hugi: Haha, no no. I meant that the pull request is waiting to happen... Waiting for someone to come along. But this is actually a really simple task. Just update the form_budgetplan_guidetext_html string in config/locales/en.yml to tell people to put one item per line in the budget.

[hspeijer]

Hugi: Use DKK. That should be made more clear.