Options that tell ProGuard to keep some classes and members (fields and methods) are described in the official documentation: http://proguard.sourceforge.net/manual/usage.html#keepoptions.
You can tell ProGuard to preserve class members using:
-keepclassmembers class_specification
Your class_specification
will look like:
class * {
@java.lang.SuppressWarnings <methods>;
}
So, in your proguard.cfg
you should have something like this:
-keepclassmembers class * {
@java.lang.SuppressWarnings <methods>;
}