Enables RC markings in nuget packages. (#3216)

diff --git a/Tools/nuget/make_pkg.proj b/Tools/nuget/make_pkg.proj
index cb124ab..0f3f8bd 100644
--- a/Tools/nuget/make_pkg.proj
+++ b/Tools/nuget/make_pkg.proj
@@ -14,6 +14,7 @@
 
     <PropertyGroup>
         <NuspecVersion>$(MajorVersionNumber).$(MinorVersionNumber).$(MicroVersionNumber)</NuspecVersion>
+        <NuspecVersion Condition="$(ReleaseLevelName) != ''">$(NuspecVersion)-$(ReleaseLevelName)</NuspecVersion>
         <SignOutput>false</SignOutput>
         <TargetName>$(OutputName).$(NuspecVersion)</TargetName>
         <TargetExt>.nupkg</TargetExt>