Add subscription selection instructions to get started.

[sptramer]

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[ghost]

OPS Build status updates of commit 0df8394:

🕙 Preparing: average preparing time is 2 min(s) 2 sec(s)

[ghost]

OPS Build status updates of commit 0df8394:

🕙 Full build: average full build time is 34 min(s) 34 sec(s) [calculated from last 46 build(s)]

Note: A full build takes more time than an incremental build. This is the expected behavior of the build system.

[ghost]

OPS Build status updates of commit 0df8394:

✅ Validation status: passed

File	Status	Preview URL	Details
docs-conceptual/azps-1.4.0/get-started-azureps.md	✅Succeeded	View (azps-1.4.0)

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.