Add a draft for a maintenance guide.

Right now it's nothing more than a guess.

MAINTENANCE.md

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+This document explains how to perform the project's maintenance tasks.
+
+### Creating a new release
+
+#### Artifacts
+
+* a tag of the version number
+* an updated `CHANGELOG.md` with the latest version
+* a new [crate version](https://crates.io/crates/flate2/versions)
+
+#### Process
+
+To generated all the artifacts above, one proceeds as follows:
+
+1. `git checkout -b release-<next-version>` - move to a branch to prepare making changes to the repository. *Changes cannot be made to `main` as it is protected.*
+2. Edit `Cargo.toml` to the next package version.
+3. `cargo changelog --write` to update the `CHANGELOG.md` file with all changes from the git history. Note that it extracts only.
+   [conventional commit messages](https://keepachangelog.com/en/1.0.0/). 
+   *Adjust the version headline to the next package version from `Unreleased`*.
+   *Copy the [GitHub release notes](https://github.com/rust-lang/flate2-rs/releases) from the previous release 
+    into the previous release section in the changelog file and adjust its headings to triple `#` to fit in*.
+4. Commit all changes.
+5. `gh pr create` to create a new PR for the current branch and get it merged.
+6. `cargo publish` to create a new release on `crates.io`.
+7. `git tag <next-version>` to remember the commit.
+8. `git push --tags` to push all changes to the branch.
+9. Go to the newly created release page on GitHub and edit it by pressing the "Generate Release Notes" and the `@` button.
+