Auto merge of #6462 - flip1995:roadmap, r=flip1995,llogiq,killercup,Manishearth,oli...

Roadmap for 2021

[Rendered](https://github.com/flip1995/rust-clippy/blob/roadmap/doc/roadmap-2021.md)

This is the first time Clippy gets its own roadmap. The reason for this roadmap is, that with the Rust language growing, also Clippy is growing. With this keeping track of and implementing bigger projects gets quite hard. This roadmap should help in exactly this regard.

After having the approval of this roadmap by the Clippy team, we want to know from the community:

- What do you think in general about this roadmap?
- Are there any pain points in Clippy, that should be included here?
- What of the points listed here has the highest priority for you?

We're looking forward to getting your feedback!

changelog: Add roadmap for Clippy 2021

r? `@rust-lang/clippy`