Add documentation on command-line function to get client ID (GoogleCloudPlatform#5712)

## Description Fixes GoogleCloudPlatform#5546 ## Checklist - [x] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md) - [x] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file) - [x] **Tests** pass: `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup)) - [x] **Lint** pass: `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup)) - [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones) - [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones) - [x] Please **merge** this PR for me once it is approved. - [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: scouvreur

composer/rest/README.rst

@@ -61,14 +61,37 @@ Install Dependencies
 Samples
 -------------------------------------------------------------------------------
 
-Determine Cloud Storage path for DAGs
+Determine client ID associated with a Cloud Composer environment
 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
-.. image:: https://gstatic.com/cloudssh/images/open-btn.png
- :target: https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/GoogleCloudPlatform/python-docs-samples&page=editor&open_in_editor=composer/rest/get_dag_prefix.py,composer/rest/README.rst
+To run this sample:
 
+.. code-block:: bash
 
+ $ python get_client_id.py
+ usage: get_client_id.py [-h] project_id location composer_environment
 
+ Get the client ID associated with a Cloud Composer environment.
+
+ positional arguments:
+ project_id Your Project ID.
+ location Region of the Cloud Composer environment.
+ composer_environment Name of the Cloud Composer environment.
+
+ optional arguments:
+ -h, --help show this help message and exit
+
+In case of any issue with `get_client_id.py`, you can also get the client ID using the following command, where `AIRFLOW_URL` would be the URL of your Airflow webserver (ex. `https://*****************-tp.appspot.com`_):
+
+.. code-block:: bash
+
+ $ curl -v AIRFLOW_URL 2>&1 >/dev/null | grep -o "client_id\=[A-Za-z0-9-]*\.apps\.googleusercontent\.com"
+
+Determine Cloud Storage path for DAGs
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+.. image:: https://gstatic.com/cloudssh/images/open-btn.png
+ :target: https://console.cloud.google.com/cloudshell/open?git_repo=https://github.com/GoogleCloudPlatform/python-docs-samples&page=editor&open_in_editor=composer/rest/get_dag_prefix.py,composer/rest/README.rst
 
 To run this sample:
 
@@ -91,8 +114,4 @@ To run this sample:
  optional arguments:
  -h, --help show this help message and exit
 
-
-
-
-
-.. _Google Cloud SDK: https://cloud.google.com/sdk/
+.. _Google Cloud SDK: https://cloud.google.com/sdk/