Create README.md for the localization folder (dotnet/extensions#683)

\n\nCommit migrated from dotnet/extensions@c841b4f

Author: natemcmaster

src/Localization/README.md

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+Localization
+============
+
+These projects provide abstractions for localizing resources in .NET applications.
+
+The ASP.NET Core implementation of localization can be found in https://github.com/aspnet/AspNetCore/tree/master/src/Middleware/Localization.