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".

CONTRIBUTING.md

@@ -39,6 +39,16 @@ necessary to use it exactly:
 All three components are important: what you did, what you expected, what
 happened instead. Please use https://gist.github.com/ if your examples run long.
 
+
+## Feature requests
+
+Cargo follows the general Rust model of evolution. All major features go through
+an RFC process. Therefore, before opening a feature request issue create a
+Pre-RFC thread on the [internals][irlo] forum to get preliminary feedback.
+Implementing a feature as a [custom subcommand][subcommands] is encouraged as it
+helps demonstrate the demand for the functionality and is a great way to deliver
+a working solution faster as it can iterate outside of cargo's release cadence.
+
 ## Working on issues
 
 If you're looking for somewhere to start, check out the [E-easy][E-Easy] and
@@ -147,6 +157,11 @@ adding labels to triage issues:
 * Magenta, **B**-prefixed labels identify bugs which are **blockers**.
 
 * Light purple, **C**-prefixed labels represent the **category** of an issue.
+  In particular, **C-feature-request** marks *proposals* for new features. If
+  an issue is **C-feature-request**, but is not **Feature accepted** or **I-nominated**,
+  then it was not thoroughly discussed, and might need some additional design
+  or perhaps should be implemented as an external subcommand first. Ping
+  @rust-lang/cargo if you want to send a PR for such issue.
 
 * Dark purple, **Command**-prefixed labels mean the issue has to do with a
   specific cargo command.
@@ -183,3 +198,5 @@ adding labels to triage issues:
 [Code of Conduct]: https://www.rust-lang.org/conduct.html
 [IRC]: https://kiwiirc.com/client/irc.mozilla.org/cargo
 [`cargotest/mod.rs`]: https://github.com/rust-lang/cargo/blob/master/tests/testsuite/cargotest/mod.rs
+[irlo]: https://internals.rust-lang.org/
+[subcommands]: https://doc.rust-lang.org/cargo/reference/external-tools.html#custom-subcommands