docs: made sure that a pitfall of make_sdf is documented (#85)

src/sprite/mod.rs

@@ -352,6 +352,10 @@ impl SpritesheetBuilder {
         self
     }
 
+    /// Add metadata to indicate that all images are SDF sprites.
+    ///
+    /// You have to ensure that the sprites are created as an SDF file beforehand. See
+    /// [`Sprite::new_sdf`] for further context.
     pub fn make_sdf(&mut self) -> &mut Self {
         self.sdf = true;
         self