Add configuration options for my.cnf

The following configuration options added:

    galera_innodb_read_io_threads: 8
    galera_innodb_write_io_threads: 8
    galera_wsrep_slave_threads: 8
    galera_xtrabackup_parallel: 4
    galera_error_log_enabled: false

Change-Id: Id1185c3a38669c37dc88806b4e9955522629ce1c
Related-Bug: https://mirantis.jira.com/browse/PROD-16528
diff --git a/README.rst b/README.rst
index 872aca0..9d59c82 100644
--- a/README.rst
+++ b/README.rst
@@ -139,6 +139,11 @@
 
 - **galera_innodb_buffer_pool_size** - the default value is 3138M
 - **galera_max_connections** - the default value is 20000
+- **galera_innodb_read_io_threads** - the default value is 8
+- **galera_innodb_write_io_threads** - the default value is 8
+- **galera_wsrep_slave_threads** - the default value is 8
+- **galera_xtrabackup_parallel** - the default value is 4
+- **galera_error_log_enabled** - the default value is 'false'
 
 Usage:
 .. code-block:: yaml
@@ -146,6 +151,11 @@
     _param:
       galera_innodb_buffer_pool_size: 1024M
       galera_max_connections: 200
+      galera_innodb_read_io_threads: 16
+      galera_innodb_write_io_threads: 16
+      galera_wsrep_slave_threads: 8
+      galera_xtrabackup_parallel: 2
+      galera_error_log_enabled: true
 
 
 Usage