Update README.md

Clarify contribution options and relationship to .NET Framework docs.

README.md

@@ -1,2 +1,6 @@
-# core-docs
-Experimenting with docs.
+.NET Core Documentation
+=======================
+
+This repo contains work-in-progress documentation for .NET Core.
+
+We very much welcome contributions to help us provide a complete set of .NET Core docs sooner. Feel free to use (copy/paste) documentation from [.NET Framework docs](https://msdn.microsoft.com/library/w0x726c2.aspx) as a starting point for .NET Core docs. We will likely port higher-quality .NET Core content to the .NET Framework docs, such that investments in this repo have a broader impact than .NET Core users. We expect that [Xamarin](http://developer.xamarin.com/api/root/classlib/), [Mono](http://docs.go-mono.com/?link=root%3a%2fclasslib) and [Unity](http://docs.unity3d.com/Manual/index.html) will also use this documentation.