SUMMARY: add patchwork

For now we are not using its tracking features, i.e. we do not update
the entries there, but it serves as an index.

If we start tracking patches there properly, then we should promote it
higher in the list.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

SUMMARY.md

@@ -85,6 +85,10 @@
   - [`staging/rust-device`](https://github.com/Rust-for-Linux/linux/tree/staging/rust-device)
   - [`staging/dev`](https://github.com/Rust-for-Linux/linux/tree/staging/dev)
 
+## Patchwork
+
+  - [Patchwork](https://patchwork.kernel.org/project/rust-for-linux/list/)
+
 ## Conferences
 
   - [Kangrejos](https://kangrejos.com)