Add comment in PatchListEntry about bumping serial UID in PatchListKey

Change-Id: Ie42c8c98ac27092283160881e659b436119f61a2

gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListEntry.java

@@ -63,6 +63,8 @@ public class PatchListEntry {
   private final int insertions;
   private final int deletions;
   private final long sizeDelta;
+  // Note: When adding new fields, the serialVersionUID in PatchListKey must be
+  // incremented so that entries from the cache are automatically invalidated.
 
   PatchListEntry(FileHeader hdr, List<Edit> editList, long sizeDelta) {
     changeType = toChangeType(hdr);