I'd just start posting again. No need for an announcement. Mind you, it depends how you left. If you flounced off dramatically denouncing everyone who disagreed with you or was nasty to you in any way, you're going to get some flak when you start posting again.

If, like me, you tend to lose interest for a few months but then a subject arises that you're interested in, just start posting again. Chances are no-one will notice, although you might be accused of running scared!!