Update azure-mobile-apps-unity.md

[elamalani]

Added note about App Center which is the next generation solution for mobile app development. It provides a variety of services for Unity developers to use in their mobile apps.

[opbld34]

Docs Build status updates of commit 626b5ab:

:warning: Validation status: warnings

File	Status	Preview URL	Details
sandbox/gamedev/unity/include/uwp-known-issues.md	:warning:Warning	View	Details
sandbox/gamedev/unity/samples/include/racer_configure-easy-tables.md	:warning:Warning	View	Details
sandbox/gamedev/unity/include/unity-import.md	:warning:Warning	View	Details
sandbox/gamedev/unity/include/unity-import-2017.md	:warning:Warning	View	Details
sandbox/gamedev/unity/azure-mobile-apps-unity.md	:white_check_mark:Succeeded	View

sandbox/gamedev/unity/include/uwp-known-issues.md

• [Warning] Missing attribute: author. Add the current author's GitHub ID.
• [Warning] Missing attribute: title. Add a title string to show in search engine results.
• Line 1: [Suggestion] H1 is required. Use a single hash (#) followed by a space to create your top-level heading. NOTE: This Suggestion will become a Warning on 11/25/2019.

sandbox/gamedev/unity/samples/include/racer_configure-easy-tables.md

• [Warning] Missing attribute: author. Add the current author's GitHub ID.
• [Warning] Missing attribute: title. Add a title string to show in search engine results.
• Line 1: [Suggestion] H1 is required. Use a single hash (#) followed by a space to create your top-level heading. NOTE: This Suggestion will become a Warning on 11/25/2019.

sandbox/gamedev/unity/include/unity-import.md

• [Warning] Missing attribute: author. Add the current author's GitHub ID.
• [Warning] Missing attribute: title. Add a title string to show in search engine results.
• Line 13: [Suggestion] H1 is required. Use a single hash (#) followed by a space to create your top-level heading. NOTE: This Suggestion will become a Warning on 11/25/2019.

sandbox/gamedev/unity/include/unity-import-2017.md

• [Warning] Missing attribute: author. Add the current author's GitHub ID.
• [Warning] Missing attribute: title. Add a title string to show in search engine results.
• Line 13: [Suggestion] H1 is required. Use a single hash (#) followed by a space to create your top-level heading. NOTE: This Suggestion will become a Warning on 11/25/2019.

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