Add README.md for generator-aspnetcore-spa. Fixes aspnet#308.

Author: SteveSandersonMS

templates/package-builder/src/yeoman/README.md

@@ -0,0 +1,12 @@
+Generates ASP.NET Core projects for single-page applications.
+
+Can generate projects with:
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+ * Angular 2
+ * Knockout
+ * React
+ * React with Redux
+
+For more information about features and usage, see [this blog post](http://blog.stevensanderson.com/2016/05/02/angular2-react-knockout-apps-on-aspnet-core/).
+
+To learn more about the underlying technologies, or to report any issues, see [this project's GitHub repo](https://github.com/aspnet/JavaScriptServices).