doc: remember to tag wasi-libc during a release

This change documents a final step called out in [wasi-libc#461]: tag
the wasi-libc repository with the same wasi-sdk version tag.

[wasi-libc#461]: https://github.com/WebAssembly/wasi-libc/issues/461

RELEASING.md

@@ -54,7 +54,16 @@ To publish a new version of `wasi-sdk` as a GitHub release:
 
 6. Publish the release; the previous step only creates a draft. Follow the link
    in the previous step or navigate to the GitHub [releases] to review the
-   description, commit, tag, and assets before clicking "Publish"
+   description, commit, tag, and assets before clicking "Publish."
+
+7. Remember to tag the wasi-libc repository with the new `$TAG` version.
+
+   ```shell script
+   git submodule status -- src/wasi-libc  # grab $WASI_LIBC_COMMIT from the output
+   cd $WASI_LIBC_REPO_DIR
+   git tag $TAG $WASI_LIBC_COMMIT
+   git push origin $TAG
+   ```
 
 [^cache]: Here is an example of how to clear a cache with the GitHub CLI: