In your config.xml, you need to add the following preference
<preference name="splashscreen" value="foo" />
Where foo is the name of the splashscreen file. Preferably a 9 patch file. Make sure to add your splashcreen files to your res/xml directory under the appropriate folders.
For Android, you also have to edit your projects main java file. You must add a second parameter representing a time delay to your super.loadUrl.
To install this plugin, follow the Command-line Interface Guide.
If you are not using the Cordova Command-line Interface, follow Using Plugman to Manage Plugins.