Update README.md

Author: PPPDUD

README.md

@@ -10,3 +10,8 @@ The entire codebase is 25 lines long, excluding comments and empty lines. Compar
 You can benchmark `homemade_yes` by running `./homemade_yes | pv -N "Homemade yes" -r -t > /dev/null`.
 
 On my build machine, it can obtain a throughput of about ~3.5 gigabytes per second, which is comparable to GNU yes (v9.5). On a nice laptop from the past ~5 years and Xubuntu 25.04, I was able to achieve ~4.2 gigabytes per second, whereas GNU yes usually hovered around ~4.0 gigabytes per second. 
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+## POSIX-only version
+I have also written a POSIX version of `yes` at `src/bin/posix-only.rs`. This version does not include the `--version` and `--help` arguments, simplifying the code massively.
+
+In all other respects, it should act roughly identical to the GNU implementation from `src/main.rs`. If it doesn't, please file an issue so that I can fix it.