RELEASES.md: document release process and checklist (#4633)

This should hopefully be clear enough that someone else than me could do
an egui release at some point, if needed.


[Rendered](https://github.com/emilk/egui/blob/emilk/releases.md/RELEASES.md)

RELEASES.md

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+# Releases
+## Cadence
+We don't have a regular cadence, but there is usually a new major release every two months or so.
+
+Often a major release is followed by one or two patch releases within a week or two.
+
+## Versioning
+All crates under the [`crates/`](crates/) folder are published in lock-step, with the same version number. This means that we won't publish a new breaking change of a single crate without also publishing all other crates. This also means we sometimes do a new release of a crate even though there are no changes to that crate.
+
+The only exception to this are patch releases, where we sometimes only patch a single crate.
+
+## Governance
+Releases are generally done by [emilk](https://github.com/emilk/), but the [rerun-io](https://github.com/rerun-io/) organization (where emilk is CTO) also has publish rights to all the crates.
+
+
+## Rust version policy
+Our Minimum Supported Rust Version (MSRV) is always _at least_ two minor release behind the latest Rust version. This means users of egui aren't forced to update to the very latest Rust version.
+
+We don't update the MSRV in a patch release, unless we really, really need to.
+
+
+# Release process
+## Patch release
+* [ ] Make a branch off of the latest release
+* [ ] cherry-pick what you want to release
+* [ ] run `cargo semver-checks`
+
+## Optional polish before a major release
+* [ ] improve the demo a bit
+* [ ] see if you can make web demo WASM smaller
+* [ ] `./scripts/docs.sh`: read and improve documentation of new stuff
+* [ ] `cargo update`
+* [ ] `cargo outdated` (or manually look for outdated crates in each `Cargo.toml`)
+* [ ] `cargo machete`
+
+## Release testing
+* [ ] `cargo r -p egui_demo_app` and click around for while
+* [ ] `./scripts/build_demo_web.sh --release`
+  - check frame-rate and wasm size
+  - test on mobile
+  - test on chromium
+  - check the in-browser profiler (build without `--release` to get profile data)
+* [ ] check the color test
+* [ ] update `eframe_template` and test
+* [ ] update `egui_tiles` and test
+* [ ] test with Rerun
+* [ ] `./scripts/check.sh`
+* [ ] check that CI is green
+
+## Preparation
+* [ ] run `scripts/generate_example_screenshots.sh` if needed
+* [ ] write a short release note that fits in a tweet
+* [ ] record gif for `CHANGELOG.md` release note (and later twitter post)
+* [ ] update changelogs using `scripts/generate_changelog.py --write`
+  - For major releases, always diff to the latest MAJOR release, e.g. `--commit-range 0.27.0..HEAD`
+* [ ] bump version numbers in workspace `Cargo.toml`
+
+## Actual release
+I usually do this all on the `master` branch, but doing it in a release branch is also fine, as long as you remember to merge it into `master` later.
+
+* [ ] `git commit -m 'Release 0.x.0 - summary'`
+* [ ] `cargo publish` (see below)
+* [ ] `git tag -a 0.x.0 -m 'Release 0.x.0 - summary'`
+* [ ] `git pull --tags ; git tag -d latest && git tag -a latest -m 'Latest release' && git push --tags origin latest --force ; git push --tags`
+* [ ] merge release PR or push to `master`
+* [ ] check that CI is green
+* [ ] do a GitHub release: https://github.com/emilk/egui/releases/new
+  * Follow the format of the last release
+* [ ] wait for documentation to build: https://docs.rs/releases/queue
+
+###  `cargo publish`:
+```
+(cd crates/emath         && cargo publish --quiet)  &&  echo "✅ emath"
+(cd crates/ecolor        && cargo publish --quiet)  &&  echo "✅ ecolor"
+(cd crates/epaint        && cargo publish --quiet)  &&  echo "✅ epaint"
+(cd crates/egui          && cargo publish --quiet)  &&  echo "✅ egui"
+(cd crates/egui_plot     && cargo publish --quiet)  &&  echo "✅ egui_plot"
+(cd crates/egui-winit    && cargo publish --quiet)  &&  echo "✅ egui-winit"
+(cd crates/egui_extras   && cargo publish --quiet)  &&  echo "✅ egui_extras"
+(cd crates/egui-wgpu     && cargo publish --quiet)  &&  echo "✅ egui-wgpu"
+(cd crates/egui_demo_lib && cargo publish --quiet)  &&  echo "✅ egui_demo_lib"
+(cd crates/egui_glow     && cargo publish --quiet)  &&  echo "✅ egui_glow"
+(cd crates/eframe        && cargo publish --quiet)  &&  echo "✅ eframe"
+```
+
+## Announcements
+* [ ] [twitter](https://x.com/ernerfeldt/status/1772665412225823105)
+* [ ] egui discord
+* [ ] [r/rust](https://www.reddit.com/r/rust/comments/1bocr5s/announcing_egui_027_with_improved_menus_and/)
+* [ ] [r/programming](https://www.reddit.com/r/programming/comments/1bocsf6/announcing_egui_027_an_easytouse_crossplatform/)
+* [ ] [This Week in Rust](https://github.com/rust-lang/this-week-in-rust/pull/5167)
+
+## After release
+* [ ] publish new `eframe_template`
+* [ ] publish new `egui_tiles`