Adding .NET standard 2.0 to supported frameworks

Author: hiranya911

FirebaseAdmin/FirebaseAdmin/FirebaseAdmin.csproj

@@ -2,8 +2,8 @@
 
   <PropertyGroup>
     <Version>1.6.0</Version>
-    <TargetFrameworks>netstandard1.5;net45</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.5</TargetFrameworks>
+    <TargetFrameworks>netstandard1.5;netstandard2.0;net45</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.5;netstandard2.0</TargetFrameworks>
     <LangVersion>latest</LangVersion>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <AssemblyOriginatorKeyFile>../../FirebaseAdmin.snk</AssemblyOriginatorKeyFile>