You’ll notice that when a dependent clause precedes an independent clause, there is a comma between the two, indicating the beginning of the main (independent) clause. However, when the independent clause comes first, there is no need to separate the two clauses with a comma.
Read more at http://grammar.yourdictionary.com/parts-of-speech/conjunctions/subordinating-conjunctions.html#VzFBcTC4MYFRpeeR.99