add page on radial gradients

content/howto/radial-gradient.md

@@ -0,0 +1,17 @@
+---
+title: apply a radial gradient
+id: batch-edit
+weight: 70
+draft: false
+author: "people"
+---
+
+darktable does not have radial gradients, but you can get a very similar effect by
+[_masking_](../darkroom/masking-and-blending/masks/drawn.md)
+with a circle or ellipse which you adjust to a very small size with a
+very large feather to get a gradual drop-off in intensity in all
+directions from a central point.
+
+It is also possible to curve a gradient mask, but this is more
+appropriate for matching a curved line in the image than for emulating
+a point source for an effect.