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/