message: Add suggestion to install MSBuild project tools extension to Known Issues
diff: ## Known Issues in 1.13.0
  * There currently is no completion support for package references in csproj files. ([#1156](https://github.com/OmniSharp/omnisharp-vscode/issues/1156))
  + * As an alternative, consider installing the [MSBuild Project Tools](https://marketplace.visualstudio.com/items?itemName=tintoy.msbuild-project-tools) extension by @tintoy.
  ## 1.13.0 _(Not Yet Released)_
  