add instructions for generating rewards data for prenight

docs/usage.rst

@@ -53,3 +53,31 @@ You can also supply an initial set of data files to show on startup:
     > --scheduler /sdf/data/rubin/user/neilsen/devel/schedview/schedview/data/sample_scheduler.pickle.xz \
     > --rewards /sdf/data/rubin/user/neilsen/devel/schedview/schedview/data/sample_rewards.h5 \
     > --port 8080
+
+The (optional) rewards data, used in the "Rewards plot" tab, can be generated
+by adding an extra option to ``sim_runner`` when running the simulation that
+creates the opsim database being examined.
+For example, to return the data when running ``sim_runner``:
+
+::
+
+    >>> from rubin_sim.scheduler import sim_runner
+    >>> observatory, scheduler, observations, reward_df, obs_rewards = sim_runner(
+    ...     observatory,
+    ...     scheduler,
+    ...     mjd_start=mjd_start,
+    ...     survey_length=night_duration,
+    ...     record_rewards=True,
+    ... )
+
+The returned ``reward_df`` and ``obs_rewards`` data can then be saved to an `h5`
+file that can then be loaded by ``prenight``:
+
+::
+
+    >>> rewards_fname = "my_rewards.h5"
+    >>> reward_df.to_hdf(rewards_fname, "reward_df")
+    >>> obs_rewards.to_hdf(rewards_fname, "obs_rewards")
+
+To be valid, the rewards data *must* be generated by the same execution of
+``sim_runner`` that generates the opsim database being examined.