Update project.json to include the .NET SDK. (firebase#339)

The .NET SDK was missing from project.json, so this change simply adds it.

Author: micahstairs

.opensource/project.json

@@ -7,6 +7,7 @@
     "content": "README.md",
     "pages": [],
     "related": [
+        "firebase/firebase-admin-dotnet",
         "firebase/firebase-admin-go",
         "firebase/firebase-admin-node",
         "firebase/firebase-admin-python"