Add proguard rules for parceler

according to http://parceler.org/
fixes issue where parceler can't find parcelable class

mastodon/proguard-rules.pro

@@ -47,3 +47,8 @@
 -keep class org.joinmastodon.android.AppCenterWrapper { *; }
 
 -keepattributes LineNumberTable
+
+# Parceler library
+-keep interface org.parceler.Parcel
+-keep @org.parceler.Parcel class * { *; }
+-keep class **$$Parcelable { *; }