java.io.IOException: You have to specify '-keep' options for the shrinking step.

执行Maven Install打包的时候，出现以下错误信息：

[proguard] java.io.IOException: You have to specify ‘-keep‘ options for the shrinking step.
 [proguard]     at proguard.SeedPrinter.write(SeedPrinter.java:60)
 [proguard]     at proguard.ProGuard.printSeeds(ProGuard.java:265)
 [proguard]     at proguard.ProGuard.execute(ProGuard.java:108)
 [proguard]     at proguard.ProGuard.main(ProGuard.java:538)

这是因为没有指定任何的“-keep”选项，例如：

-keep public class * {
    public protected *;
}