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)

- [x] add OAuth doc
- [x] one more section about getting some basic data for the current user?
- [x] proofread
- [x] update README to link to docs section