It depends on your intended audience, really.
You mention the birthright setting a bit more in your foreword (which I haven't added for any of the editors to look through), but if you want to get a hold of people who haven't really played Birthright before, perhaps a short passage would be a good idea.
On the other hand, if you intend the RG to be a supplementary spin-off of the original setting, which people can either use alongside the canon material if they want a more thourough game (which I believe is presently the case), or use as the basis for their very own setting (which I, as you know, have been intending) you won't really need to explain all that much, because then people would either already be familiar with the setting, or having it nearby to read.