Update README.md

I was confused about some of the command line flags the first time I launched labelme. I have added some information clarifying these flags to hopefully help future users avoid the confusion that I had.

README.md

@@ -160,6 +160,12 @@ For more advanced usage, please refer to the examples:
 * [Instance Segmentation Example](examples/instance_segmentation)
 * [Video Annotation Example](examples/video_annotation)
 
+### Command Line Arguemnts
+- `--output` specifies the location that annotations will be written to. If the location ends with .json, a single annotation will be written to this file. Only one image can be annotated if a location is specified with .json. If the location does not end with .json, the program will assume it is a directory. Annotations will be stored in this directory with a name that corresponds to the image that the annotation was made on.
+- The first time you run labelme, it will create a config file in `~/.labelmerc`. You can edit this file and the changes will be applied the next time that you launch labelme. If you would prefer to use a config file from another location, you can specify this file with the `--config` flag.
+- Without the `--nosortlabels` flag, the program will list labels in alphabetical order. When the program is run with this flag, it will display labels in the order that they are provided.
+- Flags are assigned to an entire image. [Example](examples/classification)
+- Labels are assigned to a single polygon. [Example](examples/bbox_detection)
 
 ## FAQ