Should we add information about this site on the Sitecore tags of StackOverflow to let people know we are here?
In reality we simply have no cause to solicit content from Stack Overflow by usurping a part of their scope to this site. Sitecore programming is still on topic on Stack Overflow, so (before it comes up), please do not solicit content or users by posting in tags or in comments on Stack Overflow; that is not allowed.
See Respect the community - your own, and others’ (emphasis mine)
As ASURA explains, its probably a good thing to wait before we go public. And when we do, there will still be users favoring StackOverflow for a while, before realizing that there are better answer on this community.
Reading the link provided by @Robert Cartaino I understand we shouldn't edit the tags, but if we see questions related to Sitecore where we think could be resolved by our site, it's Ok to add a comment suggesting to ask it here once we are generally available.