Add contributor's guide for chores

CONTRIBUTING.md

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+Contributor's Guide
+===================
+
+I don't yet have any specific requirements for new contributors
+other than the ones that Github implicitely provides.
+
+This document acts as a cookbook for how to do some specific (mundane) tasks.
+
+## Updating Dependencies
+
+Make sure the working tree is clean before starting this.
+Then, basically run the following commands:
+
+    go get -u ./...
+    go mod tidy
+    go mod vendor
+
+And then build and test to ensure everything is OK:
+
+    go build ./...
+    go test ./...
+
+Ensure that the CI is using the same version of Go as is defined in `go.mod`:
+
+    $EDITOR .github/workflows/ci.yaml
+
+Create a new commit with all the changes:
+
+    git add .
+    git commit -m "Update dependencies"
+
+## Creating a Release
+
+ 1. Ensure that the modules have been updated.
+
+ 2. Update `version.go`, especially for these metadata items:
+
+    - version (X.Y.Z)
+    - date (YYYY-MM-DD)
+    - copyright (update to include current year if necessary)
+
+ 3. Add a new entry to `NEWS.md`.
+    (The same text will be used later when we create a Github release.)
+
+ 4. Commit these changes:
+
+        git add .
+        git commit -m "Release version X.Y.Z"
+
+    However, do not push this yet. The working directory should be clean.
+
+ 6. Create an archive for testing.
+
+        git archive --prefix=repoctl-X.Y.Z/ -o ../repoctl.aur/repoctl-X.Y.Z.tar.gz HEAD
+
+    Here we refer to `repoctl.aur` for the first time. This is just the
+    [repository](https://aur.archlinux.org/packages/repoctl) that contains `PKGBUILD` for AUR.
+    If you have the rights for pushing to this, then good, otherwise you
+    will need to contact the owner of that package on AUR.
+
+ 7. In the AUR directory, you need to update the `PKGBUILD` file, in particular
+    for the new version:
+
+    - version (set to X.Y.Z)
+    - release (reset to 1)
+    - md5sum (update for source archive)
+
+    You can get the checksum with: 
+
+        md5sum repoctl-X.Y.Z.tar.gz
+
+    When this is ready, you can update `.SRCINFO`:
+
+        makepkg --printsrcinfo > .SRCINFO
+
+    There should be two files modified: `PKGBUILD` and `.SRCINFO`.
+    Now we can build the package and check that it works:
+
+        makepkg
+        sudo pacman -U repoctl-X.Y.Z-1.pkg.tar.zst
+
+    Do some rudimentary tests to ensure that everything is working
+    as expected:
+
+        which repoctl    # should be /usr/bin/repoctl
+        repoctl version  # should match current date and version
+
+    If everything is OK up until this point, then commit the changes:
+
+        git commit -a -m "Update repoctl to version X.Y.Z"
+
+    But hold off on pushing.
+
+ 8. In the main repository, push HEAD and tags:
+
+        git push
+        git push --tags
+
+ 9. In the Github Releases page, draft a new release.
+
+    - Copy the notes from `NEWS.md` into the description.
+    - Upload the source archive that we created in step 6.
+    - Create the release.
+
+10. Back in the AUR repository for repoctl, clean the working tree and
+    ensure that everything can be built cleanly from scratch:
+
+        git clean -xdf
+        makepkg
+
+    If this works out fine, then push the commit to update the AUR entry:
+
+        git push
+
+And that is how a release is made.