docs(readme): Add note to build solution
docs(readme): Add note to build solution

This helps avoid missing assembly errors.
plecong/graphql-dotnet,dNetGuru/graphql-dotnet,graphql-dotnet/graphql-dotnet,joemcbride/graphql-dotnet,dNetGuru/graphql-dotnet,alexmcmillan/graphql-dotnet,plecong/graphql-dotnet,joemcbride/graphql-dotnet,graphql-dotnet/graphql-dotnet,graphql-dotnet/graphql-dotnet,alexmcmillan/graphql-dotnet