{Doc} How to Request a Feature in CLI (#13666)

* {Doc} Service Onboarding Guide

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* Update doc/service_onboarding_guide.md

Co-authored-by: Jiashuo Li <jiasli@microsoft.com>

* Update doc/service_onboarding_guide.md

Co-authored-by: Jiashuo Li <jiasli@microsoft.com>

* Update doc/service_onboarding_guide.md

Co-authored-by: Jiashuo Li <jiasli@microsoft.com>

* 'update'

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* Update doc/how_to_request_a_feature_in_cli.md

Co-authored-by: Jiashuo Li <jiasli@microsoft.com>

Co-authored-by: Jiashuo Li <jiasli@microsoft.com>

doc/how_to_request_a_feature_in_cli.md

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+How to Request a Feature in CLI
+======
+
+This article aims to provide a guide for service team to request a feature in Azure CLI.
+
+We encourage you to contact Azure CLI team early once the swagger PR is merged and the service is released so that the work in CLI can be planned in early time. We officially support three kinds of communication channels, namely Github, email and Teams. The email address of Azure CLI is azpycli@microsoft.com.
+
+The only action you need to take is opening an issue at [Service Team Support Request](https://github.com/Azure/azure-cli/issues/new?assignees=&labels=&template=Service_team_request.md&title=). Fill in the blanks of the issue template as detailed as possible to help Azure CLI team understand the feature request without difficulties and deliver the feature as expected.
+
+Information required from service team:
+
+1. Resource provider. What is the Azure resource provider your feature is part of?
+2. Description of feature or work requested. Provide a brief description of the feature or work requested. A link to conceptual documentation may be helpful too.
+3. Minimum API version required. What is the minimum API version of your service required to implement your feature?
+4. Swagger link. Provide a link to the location of your feature(s) in the REST API specs repo. If your feature(s) has corresponding commit or pull request in the REST API specs repo, provide them. This should be on the master branch of the REST API specs repo.
+5. Target date. If you have a target date for releasing this feature/work, please provide it. While we can't guarantee these dates, it will help us prioritize your request against other requests.
+6. Where do you want the feature to locate? A command module or an extension? Here is a [Comparision](https://github.com/Azure/azure-cli/blob/dev/doc/onboarding_guide.md#extension-vs-module).
+7. Status of the service. Is it in preview or GA?
+8. Examples of the feature. Usually REST API specs repo should contain examples. An independent documentation or tutorial is also OK.  
+9. Prerequisite to use this feature. Whitelisting subscription? Registering the feature for subscription? Is it available in public Azure?
+
+The overall workflow is:
+1. Swagger and service ready
+2. Azure Python SDK ready (PyPI)
+3. Azure CLI commands ready
+
+```
+ ---------      ------------      -----------
+|         |    |            |    |           |
+| Service | -> | Python SDK | -> | Azure CLI |
+|         |    |            |    |           |
+ ---------      ------------      -----------
+```
+
+This is the way that Azure CLI collaborates with service team. The swagger PR should be merged and the service should be released before asking Azure CLI support.
+
+Feel free to contact Azure CLI team at any time through any channels. We are passionate to build the world-class cloud product.