Merge pull request #2530 from asb3993/amburns-projectsdk-note

Adding a note to indicate that feature isn't supported

Author: PRMerger6

docs/msbuild/how-to-use-project-sdk.md

@@ -54,6 +54,9 @@ During evaluation of the project, [!INCLUDE[vstecmsbuild](../extensibility/inter
 
     An implicit import is added to the top and bottom of the project as discussed above.  The format of the `Sdk` attribute is `Name[/Version]` where Version is optional.  For example, you can specify `My.Custom.Sdk/1.2.3`.
 
+    > [!NOTE]
+    > This is currently the only supported way to reference a project SDK in Visual Studio for Mac.
+
 2. Use the top-level `<Sdk/>` element:
 
     ```xml