add a CONTRIBUTING.md that links to runtimes file

src/SourceBuild/content/CONTRIBUTING.md

@@ -0,0 +1,7 @@
+Contributing
+============
+
+See [dotnet/runtime Contributing](./src/runtime/CONTRIBUTING.md) for information about coding styles, source structure, making pull requests, and more.
+
+At this time, the VMR will not accept any changes and is a read-only mirror of the development repositories only.
+Please, make the changes in the respective development repositories (e.g., [dotnet/runtime](https://github.com/dotnet/runtime) or [dotnet/sdk](https://github.com/dotnet/sdk)) and they will get synchronized into the VMR automatically.

src/VirtualMonoRepo/source-mappings.json

@@ -21,10 +21,6 @@
             "source": "src/installer/src/SourceBuild/content",
             "destination": ""
         },
-        {
-            "source": "src/runtime/CONTRIBUTING.md",
-            "destination": ""
-        },
         {
             "source": "src/installer/eng/common",
             "destination": "eng/common"