Add MAINTENANCE.md

This document describes common workflows which should help current and new maintainers alike.

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
+* 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. `gh pr create` to create a new PR for the current branch and **get it merged**.
+4. `cargo publish` to create a new release on `crates.io`.
+5. `git tag <next-version>` to remember the commit.
+6. `git push --tags` to push the new tag.
+7. Go to the newly created release page on GitHub and edit it by pressing the "Generate Release Notes" and the `@` button. Save the release.
+