Add a GraphQL Mutation with an `input` object to examples

[mntlty]

I noticed that there are no examples of Mutations in https://github.com/cli/go-gh/blob/trunk/example_gh_test.go

I think this is important for a few reasons:

• there's no mention of Mutate in the examples or in the readme. including it in the examples may make it more discoverable
• unlike queries, many of the GitHub GraphQL mutations take an input object, for example the addStar mutation and its input. because this is a nested object, it has a different format than queries such as https://github.com/cli/go-gh/blob/da64a13f361b2a1b9a1eb5358b7d7eee7506f6a2/example_gh_test.go#L125 Figuring out the right Go syntax took me some time even when I had a properly formed GraphQL mutation to compare it to

If you agree, I'd be happy to open a PR to update, and would like to know:

1. is addStar a good reference, or is there another that looks better?
2. for variables, would it be better to use https://github.com/shurcooL/githubv4 or to use only standard types?
3. should the Readme also mention mutations, or is it sufficient to have them in the examples?

[mislav]

Great idea! Go ahead please.

1. addStar seems fine! Any simple mutation would do.
2. We like to use githubv4 because it validates the input types.
3. No, the README doesn't need to showcase mutations.