docs: add a contributing guide

Codeberg became canonical for issues, PRs and releases, but there was
nothing telling a contributor how any of it works — no CONTRIBUTING.md at
all, and no contributor entry point in the README.

The workflow it documents is issue-first: a feature needs a yes-or-no
before it needs code, since whether Calendula should do a thing is the one
decision a patch cannot make. Both features and bugs get a milestone, and
that milestone names the branch a PR targets (2.18.0 -> release/v2.18.0),
because main is a release trigger rather than a staging area. Typo and docs
fixes skip straight to a PR — an issue-per-typo helps nobody.

Two traps get their own sections because both waste a contributor's whole
afternoon: translations never go through PRs (Weblate owns every values-*
file, German included, and overwrites hand-edits on the next sync), and a
clone without --recurse-submodules cannot configure at all, since
floret-kit is a composite build compiled from source.

The rules section is the invariants that actually turn into review
comments — no network, no second database, no hand-patching UI state after
a write, domain/ free of Android imports, JVM-first tests, and the
reproducible-build flags that the official F-Droid repo depends on.

README.md

@@ -105,10 +105,26 @@ without reinstalling. Or build from source — see below.
 
 ## 📚 Documentation
 
+- **[Contributing](CONTRIBUTING.md)** — how to report, propose, and patch
 - **[Building from source](docs/BUILDING.md)** — requirements and Gradle tasks
 - **[Architecture](docs/ARCHITECTURE.md)** — the layered design and key pipelines
 - **[Roadmap](.planning/ROADMAP.md)** — what's shipped and what's next
 
+## 🤝 Contributing
+
+Bug reports, ideas, and patches are all welcome on
+**[Codeberg](https://codeberg.org/jlmakiola/calendula/issues)**.
+
+The short version: **start with an issue.** Features get a yes-or-no before they
+get code, and both features and bugs are assigned a milestone whose
+`release/vX.Y.Z` branch your pull request then targets. Typo and docs fixes can
+skip straight to a pull request. Translations don't go through pull requests at
+all — [Weblate owns them](#-translations).
+
+Read **[CONTRIBUTING.md](CONTRIBUTING.md)** before writing code: it covers the
+workflow, the build (note the `floret-kit` submodule), and the architectural
+rules a change is reviewed against.
+
 ## 🌍 Translations
 
 Calendula ships in German and English, and you're warmly invited to add your