docs: Explain how to update the changelog

Author: phansch

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Change Log
 
 All notable changes to this project will be documented in this file.
+See [Changelog Update](doc/changelog_update.md) if you want to update this
+document.
 
 ## Unreleased / In Rust Beta or Nightly
 

doc/changelog_update.md

@@ -0,0 +1,59 @@
+# Changelog Update
+
+If you want to help with updating the [changelog][changelog], you're in the right place.
+
+## When to update
+
+The changelog is ideally updated during the week before an upcoming stable
+release. Typos and other small fixes/additions are always welcome. You can find
+the release dates on the [Rust Forge][forge].
+
+Most of the time we only need to update the changelog for minor Rust releases. It's
+been very rare that Clippy changes were included in a patch release.
+
+## How to update
+
+### 1. Finding the relevant Clippy commits
+
+Each Rust release ships with its own version of Clippy. The Clippy submodule can
+be found in the [tools][tools] directory of the Rust repository.
+
+To find the Clippy commit hash for a specific Rust release you select the Rust
+release tag from the dropdown and then check the commit of the Clippy directory:
+
+TODO: Include screenshot
+
+### 2. Fetching the PRs between those commits
+
+You'll want to run `util/fetch_prs_between.sh commit1 commit2 > changes.txt`
+and open that file in your editor of choice.
+
+* `commit1` is the Clippy commit hash of the previous stable release
+* `commit2` is the Clippy commit hash of the release you want to write the changelog for.
+
+When updating the changelog it's also a good idea to make sure that `commit1` is
+already correct in the current changelog.
+
+### 3. Authoring the final changelog
+
+The above script should have dumped all the relevant PRs to the file you
+specified. It should have filtered out most of the irrelevant PRs
+already, but it's a good idea to do a manual cleanup pass where you look for
+more irrelevant PRs. If you're not sure about some PRs, just leave them in for
+the review and ask for feedback.
+
+With PRs filtered, you can start to take each PR and move the
+`changelog: ` content to `CHANGELOG.md`. Adapt the wording as you see fit but
+try to keep it somewhat coherent.
+
+The order should roughly be:
+
+1. New lints
+2. Changes that expand what code existing lints cover
+3. ICE fixes
+4. False positive fixes
+5. Suggestion fixes/improvements
+
+[changelog]: https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md
+[forge]: https://forge.rust-lang.org/
+[tools]: https://github.com/rust-lang/rust/tree/master/src/tools