You need to use JetBrains Annotations, and mark the attribute with an MeansImplicitUseAttribute. You can either reference JetBrains.Annotations.dll directly, or you can copy the annotations source code (from ReSharper / Options / Code Inspection / Code Annotations) into your solution.
If you need to annotate some external assembly you don’t own, you need to create an External Annotation file (xml) in the following folder: %ReSharperInstallDir%\Bin\ExternalAnnotations. There are plenty of examples, you can just copy some.
The external annotations file can also be in the same path as the DLL if you name it DllNameWithoutExtension.ExternalAnnotations.xml.