message: Add known issue for .NET Core SDK 2.0-preview2
diff: -## Known Issues in 1.9.0
  +## Known Issues in 1.10.0
  * When opening a .csproj-based .NET Core project in VS Code, the C# extension will not activate until a C# file is opened in the editor. ([#1150](https://github.com/OmniSharp/omnisharp-vscode/issues/1150))
  * There currently is no completion support for package references in csproj files. ([#1156](https://github.com/OmniSharp/omnisharp-vscode/issues/1156))
  +* .NET Core SDK 2.0-preview2 is not yet supported. ([#1495](https://github.com/OmniSharp/omnisharp-vscode/issues/1495))
  ## 1.10.0 (May 25, 2017)
  