Contributing: add introduction with other projects

There are many areas to contribute: newcomers may not be aware that
contributions are valuable in not just the kernel. In particular, it
is important to mention the lists of "wanted features" and the other
projects.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

src/Contributing.md

@@ -1,5 +1,21 @@
 # Contributing
 
+There are many ways to contribute to Rust for Linux. One way is to contribute to the kernel itself — the rest of this page focuses on that. But there are other ways as well:
+
+  - In Rust:
+
+    + Helping to stabilize [unstable features](Unstable-features.md) the kernel requires.
+
+    + Adding support for features we would like in the kernel: in the language, in the standard library, in the compiler, in `rustdoc`, in Clippy, in `bindgen`... Please see the various ["wanted features & bugfixes" lists](https://github.com/Rust-for-Linux/linux/issues/2) for each topic.
+
+  - Contributing to the subprojects: [`klint`](klint.md) and [`pinned-init`](pinned-init.md).
+
+  - Contributing to the [Coccinelle for Rust](Coccinelle-for-Rust.md) project.
+
+  - Contributing to the [`rustc_codegen_gcc`](rustc_codegen_gcc.md) project, which will be used by the kernel for GCC builds.
+
+  - Contributing to the [GCC Rust](https://rust-gcc.github.io) project, which eventually will provide a second toolchain for GCC builds.
+
 ## The kernel development process
 
 The Rust support is part of the Linux kernel, and thus contributing works the same way as for the latter. That implies, among other things: