docs: CHANGELOG.md and CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to DongLoRa
+
+## Adding Board Support
+
+DongLoRa is designed to make adding new boards easy:
+
+1. Create `src/board/your_board.rs` implementing the `init()` function
+2. Add a Cargo feature in `Cargo.toml` with the board's dependencies
+3. Add the board's feature/target/chip to the `Justfile` boards list
+4. `build.rs` auto-discovers the new file and generates the module selector
+
+See existing board files (`heltec_v3.rs`, `rak_wisblock_4631.rs`) as templates.
+Each board file owns all hardware init and returns peripheral bundles that the
+common tasks consume. No shared board code — each board is self-contained.
+
+## Code Style
+
+- **Clippy-clean.** `just clippy <board>` must pass with no warnings.
+- **No panics in tasks.** Use `match`/`if let`/`warn!` — never `.unwrap()` in async task code. Board init (runs once at startup) may use `.expect("reason")`.
+- **Display rendering uses `.ok()`.** Drawing is best-effort; display errors are not recoverable.
+- **Every `let _ =` gets an inline comment** explaining why the result is discarded.
+
+## Building and Testing
+
+```sh
+just check-all     # Compile-check all boards (skips unavailable toolchains)
+just build-all     # Build release firmware for all boards
+just clippy <board> # Lint a specific board
+```
+
+Xtensa boards (Heltec V3/V4) require the ESP toolchain via `espup`.
+ARM boards (RAK 4631) build with stock `rustup`.
+
+## Commits
+
+- [Conventional Commits](https://www.conventionalcommits.org/) format
+- Logically grouped (one concern per commit)
+- We use [jj](https://martinvonz.github.io/jj/) for version control