Auto merge of #3824 - phansch:adding_lints, r=phansch

Add lint writing documentation

[Rendered](https://github.com/phansch/rust-clippy/blob/adding_lints/doc/adding_lints.md)

This adds a new documentation page that explains how to write Clippy
lints. It guides the reader through creating a `foo` function lint.

I plan to iterate a bit more on the prose of some sections, but I think the
general structure is fine now, so I'm looking forward to feedback =)

One thing I'm not sure about: I felt like this is too big for CONTRIBUTING.md
so I put it into a new `doc/` directory. I can imagine having more
documentation in the future, so we might even want to create a book using
mdbook instead? Or should everything go into CONTRIBUTING.md?

Further things left to do:

- [x] Link from CONTRIBUTING.md
- [x] Remove things covered in this guide from CONTRIBUTING.md
- [x] Section about `clippy::author` attribute
- [x] Run `remark-lint` on CI over the `doc` directory and fix things

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