add notes about intro to part 04

system-design/understanding-distributed-systems/part04/README.md

@@ -1 +1,10 @@
 # Resiliency
+
+The three fundamental scalability patterns discussed so far:
+ * functional decomposition - splitting a monolithic system into independent parts which composed provide the same functionality
+ * data partitioning - splitting data across several nodes
+ * replication - duplicating data across several nodes
+
+All the patterns have one thing in common - they increase the number of moving parts in our system.
+
+