add a README

mostly based on the one used in gtk-rs

Author: bilelmoussaoui

README.md

@@ -0,0 +1,41 @@
+# gtk4-rs
+
+The `gtk4-rs` are a group of crates that aims to provide gtk4 bindings.
+
+This repository contains all the "core" crates of gtk 4. For more
+information about each crate, please refer to their `README.md` file in their directory.
+
+## Regenerating
+
+To regenerate crates using [gir], please use the `generator.py`
+file as follows:
+
+```bash
+$ python3 generator.py
+```
+
+## Development
+
+
+This repository is structured as follows:
+
+```text
+- crate/
+   |
+   |-- README.md
+   |-- Gir.toml
+   |-- Cargo.toml
+   |-- src/
+   |-- sys/
+```
+
+The `crate` is a "top" directory (so "gdk4" or "gtk4" in here for example). I listed some
+import files, let's quickly explain them:
+
+ * `README.md`: Explanations about the crate itself and eventually some details.
+ * `Gir.toml`: Used by [gir] to generate most of the crates' code.
+ * `Cargo.toml`: File describing the crate, used by `cargo` and `Rust`.
+ * `src`: Contains the source code of the crate.
+ * `sys`: Contains the 1:1 bindings of the C API.
+
+[gir]: https://github.com/gtk-rs/gir