Auto merge of #5534 - matklad:feature-workflow, r=alexcrichton

Auto merge of #5534 - matklad:feature-workflow, r=alexcrichton

Feature workflow

Docs based on todays discussion.

@dwijnand you seem to actually read the docs, so I am curious how do you feel about it? :)

We have a **lot** of feature-requests for Cargo, but we care about long-term maintainability and backwards comparability, so we stick to the conservative side of things. This leads to a situation when there are a lot of `C-feature-request` tagged issues on the repo, but only a fraction of those are "yeah, we definitely want that in Cargo", while most are "this **could** be useful, but we are not really sure if it belongs to Cargo", and we'd love to signal which issues are "it would be awesome if you implement this" and which are "you could try to implement this, but there's no guarantee that the PR will be merged".