Add stub GPDB Concourse pipeline.

- This is a placeholder Concourse CI pipeline and task. See concourse.ci
  for details on the Concourse system.

README.md

@@ -74,6 +74,11 @@ throughout the codebase, but a few larger additions worth noting:
   manual is distributed separately (see http://gpdb.docs.pivotal.io),
   and only the reference pages used to build man pages are here.
 
+* __ci/__
+
+  Contains configuration files for the GPDB continuous integration system.
+ 
+
 * __src/backend/cdb/__
 
   Contains larger Greenplum-specific backend modules. For example,

ci/gpdb_5.0.yml

+---
+jobs:
+- name: placeholder-job
+  plan:
+  - task: placeholder-task
+    config:
+      platform: linux
+      image_resource:
+        type: docker-image
+        source:
+          repository: ubuntu
+          tag: '14.04'
+      run:
+        path: echo
+        args: ["This is the gpdb-5.0 stub pipeline"]