Add/update mirror README

README.md

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+This is a read-only mirror repository for use by the Swift Package Manager.
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+To view the latest changes, github issues, and pull requests please go to the main umbrella repository [azure-sdk-for-ios](https://github.com/Azure/azure-sdk-for-ios).
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+To view the documentation and source code for this package you can also select the release version via one of the [git tags](https://github.com/Azure/SwiftPM-AzureCore/tags).