Add basic readme back in

Author: Redth

README.md

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+# Android Support / AndroidX for Xamarin.Android
+
+Xamarin creates and maintains Xamarin.Android bindings for the Google Android Support Libraries and AndroidX.
+
+## Building
+
+Building from source requires calling the cake script:
+
+```
+.\build.ps1 --target=binderate
+.\build.ps1 --target=libs
+```
+
+
+## License
+
+The license for this repository is specified in
+[LICENSE.md](LICENSE.md)
+
+The `externals` build task downloads some external dependencies from Google which are licensed under and subject to the terms of [Android Software Development Kit License Agreement](http://developer.android.com/sdk/terms.html)
+
+## Contribution Guidelines
+The Contribution Guidelines for this repository are listed in [CONTRIBUTING.md](.github/CONTRIBUTING.md)
+
+## .NET Foundation
+This project is part of the [.NET Foundation](http://www.dotnetfoundation.org/projects)