Update ProGuard documentation (#1095)

Fixes #1094

Update proguard rules from initial suggestion

Co-authored-by: Leonid Startsev <[email protected]>

Author: qwwdfsad

README.md

@@ -173,7 +173,17 @@ to add this to your `proguard-rules.pro`:
 
 ```proguard
 -keepattributes *Annotation*, InnerClasses
--dontnote kotlinx.serialization.SerializationKt
+-dontnote kotlinx.serialization.AnnotationsKt # core serialization annotations
+
+# kotlinx-serialization-json specific. Add this if you have java.lang.NoClassDefFoundError kotlinx.serialization.json.JsonObjectSerializer
+-keepclassmembers class kotlinx.serialization.json.** {
+    *** Companion;
+}
+-keepclasseswithmembers class kotlinx.serialization.json.** {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+
+# Change here com.yourcompany.yourpackage
 -keep,includedescriptorclasses class com.yourcompany.yourpackage.**$$serializer { *; } # <-- change package name to your app's
 -keepclassmembers class com.yourcompany.yourpackage.** { # <-- change package name to your app's
     *** Companion;