Document use of pbandk generated code with proguard

[garyp]

To keep all generated classes, you should be able to use:

-keep class * extends pbandk.Message { *; }
-keep class * extends pbandk.Message$Companion { *; }

If you want proguard to strip out unused generated classes but keep the names, you can use something like this:

-keepnames class * extends pbandk.Message
-keepnames class * extends pbandk.Message$Companion
-keepclassmembers class * extends pbandk.Message {
    public static ** Companion;
}

Originally posted by @garyp in #141 (comment)