update docs

docs/usage.rst

@@ -8,7 +8,7 @@ Begin by activating the conda environment::
 
     $ conda activate schedview
 
-There are two ways to start the dashboard, depending on what you want to use
+There are three ways to start the dashboard, depending on what you want to use
 as the source of data.
 One way is for users to enter arbitrary URLs or file paths from which to load
 the data. **This is insecure,** because users can point the dashboard to malicious
@@ -23,6 +23,13 @@ load data from a pre-specified directory on the host running the dashboard::
 
 In either case, the app will then give you the URL at which you can find the app.
 
+Finally, if the dashboard is running at the USDF or another LFA facility, data can
+be loaded from an S3 bucket that is already preset in the dashboard. The dashboard
+will retrieve a list of snapshots for a selected night.
+
+To start the dashbaord in LFA mode::
+     $ scheduler_dashboard --lfa
+
 Running ``prenight``
 --------------------