Recommend target.'cfg(..)'.linker for recent Cargo versions

Since Cargo [version 1.74], the linker can be selected without using
`rustflags`. This way of setting it causes less conflict with other uses
of `RUSTFLAGS`, which may need to be set outside `.cargo/config.toml`.

[version 1.74]: https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-174-2023-11-16

README.md

@@ -79,6 +79,14 @@ $ cargo install flip-link
 
 Change the linker from `rust-lld` (the default) to `flip-link` in `.cargo/config.toml`
 
+``` toml
+[target.'cfg(all(target_arch = "arm", target_os = "none"))']
+# (..)
+linker = "flip-link"
+```
+
+In versions of Cargo < 1.74, use `rustflags` to change the linker
+
 ``` toml
 [target.'cfg(all(target_arch = "arm", target_os = "none"))']
 # (..)