Document that pixa serves the first frame of an animated image (closes #361)

sneak ruled against keeping animation: too much scope and attack surface.

Model: opus-5-5

README.md

@@ -545,6 +545,10 @@ need that fetch. An image whose first bytes libvips reads as another format is
 refused with 415 too. An SVG whose first bytes pass the check for AVIF is one
 example.
 
+**Animated images.** pixa serves only the first frame of an animated GIF or WebP
+source. This holds in every output format, `orig` included. So an animated GIF
+requested as GIF comes back as a still GIF.
+
 **SVG sources.** `orig` of an SVG source is the SVG itself, unchanged, and is
 cached like any other image. It is given only at the `orig` size, since pixa
 does not resize an SVG as SVG. `orig` of an SVG at any other size is refused