Added pointer to a solution for no Go files err (#1747)

While following the instructions in this getting started guide I run into this error `package github.com/99designs/gqlgen: no Go files` which was pretty annoying to fix. Its a golang issue but for people who are unfamiliar with how the `go generate` command works in vendored projects its a blocker trying to follow the rest of this guide. It will be really nice to at least have a pointer in the guide for people to find a possible solution to the issue while going through the guide. I'm sure many folks have run into this issue given vendoring is now very popular with the latest go releases.

docs/content/getting-started.md

@@ -209,6 +209,9 @@ type Todo struct {
 
 And run `go run github.com/99designs/gqlgen generate`.
 
+>
+> If you run into this error `package github.com/99designs/gqlgen: no Go files` while executing the `generate` command above, follow the instructions in [this](https://github.com/99designs/gqlgen/issues/800#issuecomment-888908950) comment for a possible solution.
+
 Now if we look in `graph/schema.resolvers.go` we can see a new resolver, lets implement it and fix `CreateTodo`.
 ```go
 func (r *mutationResolver) CreateTodo(ctx context.Context, input model.NewTodo) (*model.Todo, error) {