examples: adds crud example

[maxchagin]

No description provided.

[coveralls]

Coverage remained the same at 91.765% when pulling 8a295fb on maxchagin:master into 2800ba0 on graphql-go:master.

[chris-ramon]

Here is better to use something like Filtering without allocating when removing items within a loop, just as general recommendation.

productsUpdated := products[:0]
for _, p := range products {
  if int64(id) != p.ID {
    productsUpdated = append(productsUpdated, p)
  }
}

[chris-ramon]

This is a really great CRUD simple example! — thanks a lot @maxchagin 👍, let's merge it an improve the recomendations in a new upcoming PRs