Add README to concourse directory (#1321)

This file explains the contract that we are drawing out for the use and maintenance of the concourse directory. It describes the organization of the directory, and what should exist there.

[#134241385]

concourse/README.md

+# Concourse Directory Contract
+
+### Directory Structure
+The Concourse directory should contain this README and three sub-directories only:
+
+* pipelines
+* tasks
+* scripts
+
+##### Pipelines Directory
+There should be four pipelines in this directory:
+
+* `pipeline.yml` the pipeline that compiles, tests, and produces installers from the master branch of gpdb.
+* `pr_pipeline.yml` which compiles and tests pull requests.
+* `concourse-upgrade.yml` which uses Concourse to upgrade itself.
+* `pipeline_tinc.yml` run TINC tests against gpdb master branch.
+
+##### Tasks Directory
+All task yamls should live in this directory.
+If a task file is not referenced in the `pipelines` directory it is considered abandoned and can be removed.
+
+There are some exceptions to this rule.
+Please do not create any more exceptions, and remove these as the occasion arises:
+
+* `package_tarball.yml` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+* `test_with_planner.yml` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+* `test_with_orca.yml` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+* `build_with_orca.yml` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+
+##### Scripts Directory
+All script files should live in this directory.
+If a script file is not referenced in any of the directories it is considered abandoned and can be removed.
+
+There are some exceptions to this rule.
+Please do not create any more exceptions, and remove these as the occasion arises:
+
+* `cpplint.py` is being used by codegen
+* `package_tarball.bash` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+* `builds/` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+* `test_gpdb.py` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+* `build_gpdb.py` is being used by [a gporca pipeline ](https://github.com/greenplum-db/gporca/blob/master/concourse/pipeline.yml)
+
+### The Concourse Deployment
+There is a `gpdb` team in the [Concourse instance](http://gpdb.ci.pivotalci.info/).
+This team should only have the `gpdb_master`, `gpdb_pr`, `gpdb_master_tinc_native`, and `concourse_upgrade` pipelines.
+
+### Updating This README
+Changes should be proposed to this contract with a PR.