Add parallel test execution section to HACKING.rst

This commit adds a section to HACKING.rst to add some things to watch
out for now that we are running tempest in parallel by default. Part
of this documentation was taken from David Kranz's mailing list post:
http://lists.openstack.org/pipermail/openstack-dev/2013-August/014472.html

Change-Id: Ie688650fc528cf849c619e9afb64ba1f6b556ff8

HACKING.rst

@@ -122,3 +122,28 @@ Guidelines
 - Consistently check the status code of responses in testcases. The
   earlier a problem is detected the easier it is to debug, especially
   where there is complicated setup required.
+
+Parallel Test Exectution
+------------------------
+Tempest by default runs its tests in parallel this creates the possibility for
+interesting interactions between tests which can cause unexpected failures.
+Tenant isolation provides protection from most of the potential race conditions
+between tests outside the same class. But there are still a few of things to
+watch out for to try to avoid issues when running your tests in parallel.
+
+- Resources outside of a tenant scope still have the potential to conflict. This
+  is a larger concern for the admin tests since most resources and actions that
+  require admin privleges are outside of tenants.
+
+- Races between methods in the same class are not a problem because
+  parallelization in tempest is at the test class level, but if there is a json
+  and xml version of the same test class there could still be a race between
+  methods.
+
+- The rand_name() function from tempest.common.utils.data_utils should be used
+  anywhere a resource is created with a name. Static naming should be avoided
+  to prevent resource conflicts.
+
+- If the execution of a set of tests is required to be serialized then locking
+  can be used to perform this. See AggregatesAdminTest in
+  tempest.api.compute.admin for an example of using locking.