Add a comment on proguard-rules.pro

Author: Dr-TSNG

api/proguard-rules.pro

@@ -1,3 +1,4 @@
+# These rules are for the framework. Module rules should refer to README.md.
 -dontwarn io.github.libxposed.annotation.**
 -keep,allowoptimization public class io.github.libxposed.api.** {
     public <fields>;