README: Add project description and usage information

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 SSVLSystemAlertVolume
 =====================
 
-TODO
-A library to access the system alert volume on Mac OS X. The alert volume 
-is/controls the percentage output volume
-for beeps and sound effects.
-is set relative to the output volume, and is used for beeps and sound effects.
+A library to access the system alert volume on Mac OS X. The alert volume is
+set relative to the output volume, and is used for beeps and sound effects.
 
-This volume level is commonly set in the System Preferences’ Sound preference pane. Under the pane’s “Sound Effects” tab, it is controlled using the “Alert volume” slider.
+This volume level is commonly set in the System Preferences’ Sound preference
+pane. Under the pane’s “Sound Effects” tab, it is controlled using the “Alert
+volume” slider.
+
+![Alert volume slider in the Sound preference pane](./Alert volume slider.png)
 
 
 ## Use
-TODO
+Include the two files “SSVLSystemAlertVolume.h” and “SSVLSystemAlertVolume.c”
+in your project directly, via submodule, or another means.
 
-[[See the example](./example/main.c) for …]
+[See the example](./example/main.c) for an illustration of how to use the
+library.
 
 
 ## License