Getting Started With Octokit doc

Found myself using the same answer for multiple StackOverflow docs, so I guess it's time to write a doc about it:

[Rendered](https://github.com/octokit/octokit.net/blob/shiftkey-patch-1/docs/getting-started.md)

- [ ] add OAuth doc
- [ ] one more section about getting some basic data for the current user?
- [ ] proofread
- [ ] update README to link to docs section
- [ ] anything else to add?