Update README.md

Added a link to the Wiki on here.

README.md

@@ -55,6 +55,7 @@ To get the latest "preview" drop, published about once per month, add the `-pre`
 * [Language Feature Status](https://github.com/dotnet/roslyn/wiki/Languages-features-in-C%23-6-and-VB-14)
 * [Language Design Notes](https://github.com/dotnet/roslyn/issues?q=label%3A%22Design+Notes%22+)
 * [FAQ](https://github.com/dotnet/roslyn/wiki/FAQ)
+* Also take a look at our [Wiki](https://github.com/dotnet/roslyn/wiki) for more information on how to contribute, what the labels on issue mean, etc.
 
 ### Contribute!