Adding section about rust-analyzer in README

- Including an example of how to create a rust-analyzer `rust-project.json` config
- Adding `rust-project.json` to `.gitignore`

Signed-off-by: Matthew Wood <mat@thepacketgeek.com>

.gitignore

@@ -8,3 +8,4 @@
 *.rmeta
 Module.symvers
 modules.order
+rust-project.json

README.md

@@ -34,3 +34,11 @@ make[1]: Leaving directory '.../linux-with-rust-support'
 For details about the Rust support, see https://github.com/Rust-for-Linux/linux.
 
 For details about out-of-tree modules, see https://www.kernel.org/doc/html/latest/kbuild/modules.html.
+
+## rust-analyzer
+
+Rust for Linux supports building a `rust-project.json` configuration for [`rust-analyzer`](https://rust-analyzer.github.io/), including for out-of-tree modules:
+
+```sh
+make -C .../linux-with-rust-support M=`pwd` rust-analyzer
+```