Allow for specifying root job directory

The finger log streamer will look in the root job dir to
find job logs.

Change-Id: Ib585e04e890cf73bd254d3ba4e7ac3cf01cd342c
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 56cc6a8..3721076 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -108,6 +108,10 @@
   commands.
   ``state_dir=/var/lib/zuul``
 
+**jobroot_dir**
+  Path to directory that Zuul should store temporary job files.
+  ``jobroot_dir=/tmp``
+
 **report_times**
   Boolean value (``true`` or ``false``) that determines if Zuul should
   include elapsed times for each job in the textual report.  Used by