docs: clarify the build process for the npm release (#1568)

It appears that `0.5.0` was built with `stage_release.sh` instead of
`stage_rust_release.py`, so add docs to clarify this and recommend
running `--version` on the release candidate to verify the right thing
was built.

codex-cli/scripts/README.md

@@ -0,0 +1,9 @@
+# npm releases
+
+Run the following:
+
+To build the 0.2.x or later version of the npm module, which runs the Rust version of the CLI, build it as follows:
+
+```bash
+./codex-cli/scripts/stage_rust_release.py --release-version 0.6.0
+```