Document how to backport changes to beta

doc/backport.md

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+# Backport Changes
+
+Sometimes it is necessary to backport changes to the beta release of Clippy.
+Backports in Clippy are rare and should be approved by the Clippy team. For
+example, a backport is done, if a crucial ICE was fixed or a lint is broken to a
+point, that it has to be disabled, before landing on stable.
+
+Backports are done to the `beta` release of Clippy. Backports to stable Clippy
+releases basically don't exist, since this would require a Rust point release,
+which is almost never justifiable for a Clippy fix.
+
+
+## Backport the changes
+
+Backports are done on the beta branch of the Clippy repository.
+
+```bash
+# Assuming the current directory corresponds to the Clippy repository
+$ git checkout beta
+$ git checkout -b backport
+$ git cherry-pick <SHA>  # `<SHA>` is the commit hash of the commit, that should be backported
+$ git push origin backport
+```
+
+After this, you can open a PR to the `beta` branch of the Clippy repository.
+
+
+## Update Clippy in the Rust Repository
+
+This step must be done, **after** the PR of the previous step was merged.
+
+After the backport landed in the Clippy repository, also the Clippy version on
+the Rust `beta` branch has to be updated.
+
+```bash
+# Assuming the current directory corresponds to the Rust repository
+$ git checkout beta
+$ git checkout -b clippy_backport
+$ pushd src/tools/clippy
+$ git fetch
+$ git checkout beta
+$ popd
+$ git add src/tools/clippy
+§ git commit -m "Update Clippy"
+$ git push origin clippy_backport
+```
+
+After this you can open a PR to the `beta` branch of the Rust repository. In
+this PR you should tag the Clippy team member, that agreed to the backport or
+the `@rust-lang/clippy` team. Make sure to add `[beta]` to the title of the PR.