mention video format support

README.md

@@ -9,7 +9,6 @@ Calculations
 ------------
 
 ```
-docker pull paulfitz/makesweet
 docker run -v $PWD:/share paulfitz/makesweet /reanimator \
   --zip templates/billboard-cityscape.zip \
   --in templates/frog.jpg \
@@ -20,3 +19,15 @@ Output
 ------
 
 ![animation](https://user-images.githubusercontent.com/118367/39386216-8f26a80e-4a41-11e8-8ae0-0d44a5a55af1.gif)
+
+Video format
+------------
+
+You can generate a video output instead of a gif by replacing:
+```
+  --gif animation.gif
+```
+with:
+```
+  --vid animation.mp4
+```