split performance in two parts and add sticky reads part. (#4131)

Descriptions of the changes in this PR:


### Motivation

After the issue on disabled sticky reads, I'm adding this to improve documentation about performance on reading.

### Changes

Master Issue: #4130

site3/website/docs/admin/bookies.md

@@ -18,8 +18,14 @@ There is no upper limit on the number of bookies that you can run in a single en
 
 ### Performance
 
+#### Disks
+
 To achieve optimal performance, BookKeeper requires each server to have at least two disks. It's possible to run a bookie with a single disk but performance will be significantly degraded.
 
+#### Sticky reads
+
+BookKeeper sticky reads enables bookie node to read entries efficiently. The sticky reads are only available when ensemble (E) size is equal to quorum write (Qw).
+
 ### ZooKeeper
 
 There is no constraint on the number of ZooKeeper nodes you can run with BookKeeper. A single machine running ZooKeeper in [standalone mode](https://zookeeper.apache.org/doc/current/zookeeperStarted.html#sc_InstallingSingleMode) is sufficient for BookKeeper, although for the sake of higher resilience we recommend running ZooKeeper in [quorum mode](https://zookeeper.apache.org/doc/current/zookeeperStarted.html#sc_RunningReplicatedZooKeeper) with multiple servers.