bindings: rust: add README.md for libgpiod crate

crates.io treats the README as landing page for a crate [1]. Since
we have none, it currently displays a blank page. Lets add at least a
little bit of info here so people can figure out what they are dealing
with.

[1] https://crates.io/crates/libgpiod

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

diff --git a/bindings/rust/libgpiod/README.md b/bindings/rust/libgpiod/README.md
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+<!--
+SPDX-License-Identifier: CC0-1.0
+SPDX-FileCopyrightText: 2023 Linaro Ltd.
+SPDX-FileCopyrightText: 2023 Erik Schilling <erik.schilling@linaro.org>
+-->
+
+# Safe wrapper around Rust FFI bindings for libgpiod
+
+[libgpiod](https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/tree/README)
+is a C library that provides an easy to use abstraction over the Linux GPIO
+character driver. This crate builds on top of `libgpiod-sys` and exports a safe
+interface to the C library.
+
+## Build requirements
+
+By default, `libgpiod-sys` builds against the libgpiod version identified via
+`pkg-config`. See the `README.md` of `libgpiod-sys` for options to override
+that.
+
+## License
+
+This project is licensed under either of
+
+- [Apache License](http://www.apache.org/licenses/LICENSE-2.0), Version 2.0
+- [BSD-3-Clause License](https://opensource.org/licenses/BSD-3-Clause)