[scroll_down] make parameter configurable, update readme

Author: jcb91

src/jupyter_contrib_nbextensions/nbextensions/scroll_down/config.yaml

@@ -5,3 +5,8 @@ Link: readme.md
 Icon: icon.png
 Main: main.js
 Compatibility: 4.x, 5.x
+Parameters:
+- name: scrollDownIsEnabled
+  description: enable scroll_down toolbar button
+  input_type: checkbox
+  default: false

src/jupyter_contrib_nbextensions/nbextensions/scroll_down/readme.md

@@ -1,3 +1,13 @@
 # ScrollDown
 
-When enabled, scrolls cells' outputs down automatically.
+When enabled, this nbextension scrolls cells' outputs down automatically as
+they are enlarged. It can be enabled and disabled using the toolbar button:
+
+![](icon.png)
+
+
+Options
+-------
+
+The enabled status is toggleable with the toolbar button, which sets the option
+option `scrollDownIsEnabled` in the notebook config section.