Moved more content from codeplex

The README.md file still referred to several pieces of CodePlex content.
This content was migrated to GitHub and the README points to the updated
location.

README.md

@@ -69,20 +69,18 @@ From the NuGet package manager console:
 * [Syntax Visualizer Tool](https://github.com/dotnet/roslyn/wiki/Syntax%20Visualizer)
 * [Roadmap](https://github.com/dotnet/roslyn/wiki/Roadmap) 
 * [Language Feature Status](https://github.com/dotnet/roslyn/wiki/Languages-features-in-C%23-6-and-VB-14)
-* [Language Design Notes](https://roslyn.codeplex.com/documentation?referringTitle=Home&ANCHOR#langdesignlinks)
+* [Language Design Notes](https://github.com/dotnet/roslyn/issues?q=label%3A%22Area-Design+Notes%22+)
 * [FAQ](https://github.com/dotnet/roslyn/wiki/FAQ)
 
 ### Contribute!
 
 Some of the best ways to contribute are to try things out, file bugs, and join in design conversations. 
 
-* [Questions, Comments, and Feedback](https://roslyn.codeplex.com/wikipage?title=Questions%2c%20Comments%2c%20and%20Feedback&referringTitle=Home)
-
-Looking for something to work on? The list of [up-for-grabs issues](https://roslyn.codeplex.com/workitem/list/advanced?keyword=grabs&status=Open%2b%28not%2bclosed%29&type=All&priority=All&release=All&assignedTo=All&component=All&reasonClosed=All&sortField=LastUpdatedDate&sortDirection=Descending&page=0) is a great place to start.
-
-* [How to Contribute](https://roslyn.codeplex.com/wikipage?title=How%20to%20Contribute&referringTitle=Home) 
+* [How to Contribute|Contributing Code]
 * [Pull requests](https://github.com/dotnet/roslyn/pulls): [Open](https://github.com/dotnet/roslyn/pulls?q=is%3Aopen+is%3Apr)/[Closed](https://github.com/dotnet/roslyn/pulls?q=is%3Apr+is%3Aclosed)
 
+Looking for something to work on? The list of [up for grabs issues](https://github.com/dotnet/roslyn/issues?q=is%3Aopen+is%3Aissue+label%3A%22Up+for+Grabs%22) is a great place to start.
+
 ### .NET Foundation
 
 This project is part of the [.NET Foundation](http://www.dotnetfoundation.org/projects) along with other