Add contribution and maintenance docs

Author: jonas-schievink

CONTRIBUTING.md

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+# Contribution and Maintenance notes
+
+## Contributing
+
+Pull Requests and issue reports are very welcome!
+
+If you are unsure whether something would make a good contribution (or you aren't sure how to
+tackle something), feel free to either open an issue to solicit feedback or chat with us at
+[`#nrf-rs:matrix.org`]. This is preferred over opening a big PR that might then not get merged.
+
+If a Pull Request has not been reviewed for a long time, feel free to ping the maintainers.
+
+## Breaking Changes
+
+Since the HAL still evolves rapidly, most releases contain breaking changes, and there is generally
+no requirement that Pull Requests avoid them.
+
+If some fix is intended to be included in a patch release, breaking changes should be avoided.
+Otherwise it can not be included in a patch release and has to wait until the next major release.
+
+Note that we do not backport bugfixes, so once any breaking change has landed, future bug fixes,
+even if not breaking changes by themselves, will only be included in the next major release.
+
+## Release process
+
+In order to release a new version of the HALs, the following steps need to be performed:
+
+* **Changelog**: Update [the changelog](./CHANGELOG.md) to list all notable changes under the `Unreleased`
+  section.
+  * You can use GitHub's "compare" feature to view all commits added since the last release. Go to
+    <https://github.com/nrf-rs/nrf-hal/releases/>, select the latest release, and click the link
+    "N commits to master since this release".
+* **Version Bump**: Determine whether the next release contains breaking changes. This informs what
+  kind of version bump is needed (minor vs. major). Then, bump the crate versions accordingly.
+  Because of the large number of crates, we use [`cargo-xtask`]-based automation to bump all version
+  numbers. Invoke it via `cargo xtask bump <new-version>` to update all version numbers to
+  `<new-version>`.
+* **Pull Request**: Open a Pull Request with the version bump and merge it once CI passes.
+* **Tag**: Run `git pull`, and tag the release by running `git tag -a -m 'vX.Y.Z' 'vX.Y.Z'`,
+  replacing `X.Y.Z` with the version you bumped to. Run `git push --tags origin`.
+* **Publish**: Publish all HAL crates to crates.io, starting with `nrf-hal-common`. For that crate,
+  you have to pass `--no-verify` to `cargo publish`, since the crate only builds with specific
+  Cargo features. The examples should not be published.
+* **GitHub Release**: Go to <https://github.com/nrf-rs/nrf-hal/releases> and click the tag you just
+  pushed. Click *Edit Tag* and paste the changelog entries for the new version (no *Release Title*
+  is necessary). Click *Publish Release*.
+* **Annoucement**: Post a link to the GitHub release in [`#nrf-rs:matrix.org`] and any other places
+  you'd like to announce it.
+
+[`cargo-xtask`]: https://github.com/matklad/cargo-xtask/
+[`#nrf-rs:matrix.org`]: https://matrix.to/#/#nrf-rs:matrix.org