Added note about budgeted cost in SFDC setup instructions

Why is this change being made?

@amy.waller - When working on the webcast/workshop instructions with @lblanchard I asked about a note in the SFDC setup instructions that mentions if the cost of the campaign is $0 we should be adding a 1 instead of a 0 in the budgeted cost field. She confirmed this is correct so I'm adding this note to the general Marketo/SFDC setup instructions. Please let us know if anything with this has changed or if this is still correct. Thanks!

cc @jennyt @lilphil @Helenadixon @ttran5

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