Add "Out-of-tree modules" page

In a recent discussion around a networking submission, it seemed
that some extra clarifications about out-of-tree modules could
be useful for developers looking to develop Rust code for
the kernel [1].

Thus add an "Out-of-tree modules" page to clarify the focus of
the Rust for Linux project, what is intended to be supported
around out-of-tree code and what kinds of patches make sense to
submit to the kernel mailing list.

Importantly, add links to other pages from the mainline kernel
documentation.

Link: https://rust-for-linux.zulipchat.com/#narrow/stream/396592-Networking/topic/NAPI.20Abstractions/near/385885376 [1]
Signed-off-by: Miguel Ojeda <[email protected]>

SUMMARY.md

@@ -9,6 +9,7 @@
   - [Unstable features](Unstable-features.md)
   - [Backporting and stable/LTS releases](Backporting-and-stable-LTS-releases.md)
   - [Third-party crates](Third-party-crates.md)
+  - [Out-of-tree modules](Out-of-tree-modules.md)
   - [Industry and academia support](Industry-and-academia-support.md)
   - [Sponsors](Sponsors.md)
 

src/Out-of-tree-modules.md

@@ -0,0 +1,13 @@
+# Out-of-tree modules
+
+The Linux kernel supports [building out-of-tree modules](https://docs.kernel.org/kbuild/modules.html). Both C and Rust modules can be developed as out-of-tree ones, and we provide a [basic template](https://github.com/Rust-for-Linux/rust-out-of-tree-module) for an out-of-tree Linux kernel module written in Rust.
+
+However, please note that the Rust for Linux project is part of the kernel and has always focused its efforts towards getting code into the mainline kernel.
+
+In particular, this means that Rust [internal APIs can be changed at any time](https://docs.kernel.org/process/4.Coding.html#internal-api-changes), just like C ones, when the need arises. Similarly, code present at any point in our different [branches](Branches.md) is not intended to form a stable base for out-of-tree development.
+
+In addition, patches submitted to the mailing list should generally focus on in-tree development efforts. In particular, Rust abstractions submitted upstream require in-tree users. Abstractions intended for out-of-tree users cannot be merged. Even if those abstractions may be obviously useful for future in-tree users, there needs to be an agreed upon in-tree user.
+
+For these reasons and others, please consider [submitting](Contributing.md) your use cases upstream — see [the importance of getting code into the mainline](https://docs.kernel.org/process/1.Intro.html#the-importance-of-getting-code-into-the-mainline).
+
+Having said that, we understand that some module development is done out-of-tree and may not be possible to upstream. Even in those cases, if your company, organization or team has a use case for Rust, please [contact us](Contact.md), since it is important to highlight those use cases early on in order to showcase the [interest from industry and academia](Industry-and-academia-support.md) in Rust.