Add RHEL policy

documents/process/rust_versions.md

@@ -56,6 +56,7 @@ An `LLVM_COMPATIBLE_NIGHTLY` is said to "work with" an LLVM version when the siz
  - It MUST be available on the GitHub Actions runners with `ubuntu-latest`
  - It SHOULD be available on the latest XCode
  - It SHOULD be available on ICU4X developer machines using nonstandard package management (more on this below)
+ - It SHOULD be available on RHEL and latest Rocky Linux via `yum`
 
 
 
@@ -111,6 +112,14 @@ Command: `brew install llvm@18` on OSX.
 
 Verification:  Visit https://formulae.brew.sh/formula/llvm#default and ensure the `llvm@N` package is listed, for version N.
 
+#### RHEL / Rocky Linux
+
+Policy: Should be available via `yum`, but policy can be overridden by TC approval in cases where there is a strong case for a new feature.
+
+Command: `yum module install llvm-toolset`
+
+Verification: Latest Rocky Linux (currently Rocky Linux 9) and RHEL should be listed on [this page](https://pkgs.org/search/?q=llvm-toolset) with an LLVM version that is equal to or greater than the desired LLVM version. It's okay if the exact LLVM version is not listed, `yum module install` allows installing older versions via explicit command.
+
 #### GitHub Actions
 
 Policy: Must be available on GitHub actions `ubuntu-latest` (i.e. ICU4X CI should be able to just `apt install` the LLVM)