Add optional setting `java_args` to the master role

Change-Id: I20760f5dc9d2a7c085ad3995a99f438eab7823b4
diff --git a/README.rst b/README.rst
index df9f3d9..09d860b 100644
--- a/README.rst
+++ b/README.rst
@@ -70,6 +70,7 @@
     jenkins:
       master:
         mode: EXCLUSIVE
+        java_args: -Xms256m -Xmx1g
         # Do not manage config.xml from Salt, use UI instead
         no_config: true
         slaves: