README update for sidebar functions

[michaelmilessmith]

The README for the sidebar functions seems to be outdated.

• widgetNamespace should list app as one of the options.
• insertBeforeWidgetId should be a string not an object

[DevlinRocha]

I noticed this too, was gonna submit a PR to fix it but apparently the maintainers here don't look at contributions so I didn't bother. =/

#1236
#1222