Merge pull request #1106 from zalando/repartitioning-docs

Repartitioning documentation

Author: adyach

docs/_documentation/using_repartitioning.md

@@ -0,0 +1,21 @@
+---
+title: Repartitioning
+position: 12
+---
+
+## Repartitioning
+
+Throughput of event type is defined by default statistic, which basically sets number of partitions for the event
+ type (although it does not represent it clearly). Number of partitions is a scaling unit for Nakadi publishing and
+  consumption. In order to change number of partitions one have to repartition event type by changing
+   read_parallelism and write_parallelism. Nakadi takes for repartitioning the maximum of read_parallelism and
+    write_parallelism.
+
+### Important caveats
+
+- Publishing events to event type with hash partition strategy will change the partitions in which they were appearing
+ before
+- Nakadi guarantees ordering per partition per batch, repartitioning event types from 1 partitions to more will break
+ total order of events
+- Repartitioning allows to only increase number of partitions
+- Consuming subscriptions are disconnected once repartitioning is finished