Updating readme to include batch mode instructions (and correction)

Author: pshriwise

README.md

@@ -30,7 +30,7 @@ pip install openmc-plotter
 From a directory containing an OpenMC model run:
 
 ```console
-$ openmc-plotter -d <path_to_openmc_model_dir>
+$ openmc-plotter <path_to_openmc_model_dir>
 ```
 
 or simply run
@@ -45,6 +45,15 @@ Once the viewer has opened, press `?` to view a variety of keyboard shortcuts.
 
 ![shortcuts](./screenshots/shortcuts.png)
 
+## Batch Mode
+
+Plot view (`.pltvw`) files can be used to generate images from the command line without
+opening the plotter GUI.
+
+```console
+$ openmc-plotter -b view1.pltvw view1.pltvw view1.pltvw
+```
+
 ## Features
 
 ### Tally Visualization