Update README.md with purpose of repo

Adding V1 of section "Microsoft AL application add-on and localization extensions for Microsoft Dynamics 365 Business Central" to the README.md file.

README.md

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+# Microsoft AL application add-on and localization extensions for Microsoft Dynamics 365 Business Central
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+The purpose of this repository is to create a collaboration platform between Microsoft and our vibrant partner channel and community for the joint development of application extensions in the AL language. It is an opportunity to enhance the channel for providing feedback and improvement suggestions on work done on extensions by Microsoft.
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+Going forward, an increasing part of our application business logic will be modularized and extracted into extensions, which will be published onto this repository; this is true for both application add-ons as well as application localizations. The core application thereby becomes thinner, better extensible and better localizable. The extracted modules become open for contributions, are replaceable in the application with substitutes, serve as starting point for verticalizations of the modules or serve as samples for extension development in general.
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+The jointly developed extensions in this repository will be published by Microsoft to App Source and will be shipped with upcoming releases of Microsoft Dynamics 365 Business Central.
 
 # Contributing