Add film in a mask and the hot-pixel repair to the README

The developing paragraph listed masks and film separately and said
nothing of photosites. It now says that a mask's sliders add to the
photograph's, the film's among them, so a sky can be burned in on the
print (0.18.1 and 6b99f67), and that hot and dead photosites are mended
before the demosaic with nothing to set (c50d96e).

README.md

@@ -31,7 +31,10 @@ capture sharpening, noise reduction, lens correction, spectral film
 simulation. Crop, straighten and correct converging verticals, spot repair,
 and local adjustments over masks the model draws — click a subject or a
 category, then paint, subtract a gradient or keep only where two selections
-agree, grow or shrink the edge. Focus peaking and a raw histogram for judging
+agree, grow or shrink the edge. A mask's sliders add to the photograph's, the
+film's among them, so a sky can be burned in on the print as a darkroom
+printer would. Hot and dead photosites are mended before the demosaic, with
+nothing to set. Focus peaking and a raw histogram for judging
 what is recoverable. Presets, with a collection shipped in the application —
 everyday corrections, and a look for each measured colour, cinema and
 black-and-white stock — and Lightroom presets imported as looks that leave a