Add userContext to README example
Add userContext to README example

alexmcmillan/graphql-dotnet,joemcbride/graphql-dotnet,graphql-dotnet/graphql-dotnet,plecong/graphql-dotnet,graphql-dotnet/graphql-dotnet,dNetGuru/graphql-dotnet,alexmcmillan/graphql-dotnet,dNetGuru/graphql-dotnet,graphql-dotnet/graphql-dotnet,joemcbride/graphql-dotnet,plecong/graphql-dotnet