Added a reference to xdcr compatibility in vbuckets page

Author: rao-shwe

modules/learn/pages/buckets-memory-and-storage/vbuckets.adoc

@@ -24,6 +24,8 @@ The number of vBuckets depends on the bucket's storage backend (the system that
 * Buckets that use the Couchstore storage engine use 1024 vBuckets on Linux and Windows.
 * Buckets that use Magma storage engine can use either 128 or 1024 vBuckets on Linux and Windows.
 You choose the number of vBuckets when you create the bucket.
+If you're using XDCR, review the xref:learn:clusters-and-availability/xdcr-overview.adoc#xdcr-compatibility[XDCR Compatibility] information
+on vBuckets for Server versions earlier than 8.0.
 
 The system distributes vBuckets evenly across the memory and storage resources of nodes that run the xref:services-and-indexes/services/data-service.adoc[Data Service].
 The bucket's data is distributed evenly across its vBuckets.