docs: rework the roadmap; GeoLibre parity and CONTRIBUTING.md in the repo

AGENTS.md

@@ -4,6 +4,9 @@ RustGIS is a fast, free and open-source GIS in pure Rust for mapping, analyzing
 geospatial data, on the desktop and in the browser. Its architecture: layered crates, everything
 is a command, a thin egui front end, headless screenshots for UI work.
 
+Before picking work, read [ROADMAP.md](ROADMAP.md) (check Underway) and
+[CONTRIBUTING.md](CONTRIBUTING.md) (what to coordinate first).
+
 ## Never crash
 - No `unwrap()`, `expect()`, `panic!`, `unreachable!`, `todo!` in non-test code; no `unsafe`
   (`unsafe_code = "forbid"`). Crate roots carry the `#![deny(clippy::unwrap_used, …)]` block.

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing to RustGIS
+
+Contributions are welcome. This page covers what to work on and how to propose it. For build,
+test, docs and release mechanics see [docs/contributing.md](docs/contributing.md); for code
+conventions see [AGENTS.md](AGENTS.md).
+
+## Most roadmap items have no issue
+
+[ROADMAP.md](ROADMAP.md) lists where RustGIS is heading. Only a few of its items link to an
+issue; **most features have no issue yet**. Don't wait for one to appear before starting. If
+something on the roadmap (or not on it) interests you, propose it and open a pull request.
+
+## How to contribute a feature
+
+1. **Check for an existing issue.** Search the
+   [issues](https://github.com/opengeos/RustGIS/issues) and the roadmap. If there is an issue,
+   comment on it to say you're taking it.
+2. **Write a short proposal** if there is no issue. It covers:
+   - the problem or workflow it serves;
+   - what you plan to build: the commands it adds, where it sits in the ribbon, which formats or
+     tools are involved;
+   - which crates it touches and whether it adds a dependency;
+   - how you will test it.
+
+   Put it in an issue with the
+   [feature request](https://github.com/opengeos/RustGIS/issues/new?template=feature_request.yml)
+   form, or in the description of a draft pull request. For changes that need coordination (see
+   below) use an issue and wait for a maintainer's reply; otherwise either works.
+3. **Open a pull request.** Draft pull requests are welcome early, to get feedback on the
+   approach before the work is finished. Link the issue (`Closes #123`) and fill in the
+   [pull request template](.github/pull_request_template.md). Before it is ready for review, it
+   must meet the rules and quality gates in [AGENTS.md](AGENTS.md).
+
+## Pick freely or coordinate first
+
+- **Pick freely**: roadmap items marked _(open)_ are self-contained. Open a pull request directly,
+  with the proposal in its description; an issue is optional.
+- **Coordinate first**: open a proposal issue and wait for a maintainer's reply before writing much
+  code if your change
+  - changes the data model (`Feature`, `Coord`, `Renderer`);
+  - adds a dependency or a crate;
+  - changes the `.rgis` project format, or an existing command's id or parameters (the CLI,
+    Python, MCP and saved scripts call commands by id);
+  - changes the plugin interface;
+  - falls under _Exploratory_ in the roadmap.
+
+Roadmap themes are not releases or an order; an item moves to Done when it lands. A
+pull request that lands a roadmap item, or an item of [GEOLIBRE_PARITY.md](GEOLIBRE_PARITY.md),
+updates that file in the same change.