Added Roslyn Debugging Steps (#2197)

* Added Roslyn Debugging Steps

Thanks @NTaylorMullen for the guide, just writing up the process for future reference.

Directory structure inspired by [roslyn](https://github.com/dotnet/roslyn/blob/master/docs/contributing/Building,%20Debugging,%20and%20Testing%20on%20Windows.md).

docs/contributing/RoslynDebugging.md

@@ -0,0 +1,16 @@
+# Debugging with experimental Roslyn bits
+Sometimes it may be necessary to make changes in [`dotnet/roslyn`](https://github.com/dotnet/roslyn), and react to the changes in this repo. The following are steps which outline the general process in using Roslyn development `dll`s with Razor Tooling.
+
+## Steps:
+1. Checkout [`dotnet/roslyn`](https://github.com/dotnet/roslyn).
+2. `./Restore.cmd`
+3. Make the desired changes in `dotnet/roslyn`.
+4. `./Build.cmd -pack`. The `-pack` option causes the creation of NuGet packages.
+5. You should see the generated packages in the `roslyn\artifacts\packages\Debug\Release` directory. Take note of the package versions (ie. `Microsoft.CodeAnalysis.Workspaces.Common.3.8.0.nupkg` => `3.8.0`).
+6. Open `aspnetcore-tooling/NuGet.config` and add the local package source `<add key="Roslyn Local Package source" value="<PATH_TO_ROSLYN_REPO>\artifacts\packages\Debug\Release" />`.
+7. Open `aspnetcore-tooling/eng/Versions.props` and update all the `Tooling_*` versions to the version noted in step 5, except for  `Tooling_MicrosoftVisualStudioLanguageServicesRazorRemoteClientPackageVersion`.
+
+## Notes:
+- If you're familiar with _Visual Studio Hives_ the `dotnet/roslyn` project uses the `RoslynDev` root suffix .
+- [Building Roslyn on Windows](https://github.com/dotnet/roslyn/blob/master/docs/contributing/Building,%20Debugging,%20and%20Testing%20on%20Windows.md)
+- [Building Roslyn on Linux and Mac](https://github.com/dotnet/roslyn/blob/master/docs/infrastructure/cross-platform.md)