Label on boundary of region plot

Here's how:

p1 = RegionPlot[{n + k > 6}, {n, 0, 4}, {k, 0, 15}, BoundaryStyle->None];
p2 = Rotate[Graphics[Text[Style["label here", FontSize->20,Background-> Transparent, Black, Bold],Scaled[{1/4, 6.5/15}]]], -ArcTan[4/15]];
Overlay[{p1, p2}]

nmax = 4; kmax = 15;
RegionPlot[{n + k > 6}, {n, 0, nmax}, {k, 0, kmax}, BoundaryStyle -> None,
  Epilog -> Text[Style["label here", 16, Bold], {1, 5.5}, Left, {kmax, - nmax}]]