Enable publishing of features/readonly-ref artefacts.

build/config/PublishData.json

@@ -13,6 +13,13 @@
             "nuget": [ "https://dotnet.myget.org/F/roslyn/api/v2/package" ],
             "vsix": [ "https://dotnet.myget.org/F/roslyn/vsix/upload" ],
             "channels": [ "publish-test" ]
+        },
+        "features/readonly-ref": {
+            "nugetKind": "PerBuildPreRelease",
+            "version": "2.6.*",
+            "nuget": [ "https://dotnet.myget.org/F/roslyn/api/v2/package" ],
+            "vsix": [ "https://dotnet.myget.org/F/roslyn/vsix/upload" ],
+            "channels": [ "dev15.5", "dev15.5p1" ]
         }
     },
     "releases": {