Update HOWTO.md (#4168)

Added link of HOWTO-zh.md .
**This file is in PR right now, please consider it

HOWTO.md

+Read this in other languages:[中文](HOWTO-zh.md)
+
 Welcome to Free-Programming-Books! We welcome new contributors; even those making their very first pull request on Github. If you're one of those, here are some resources that might help:
 
 * [About Pull Requests](https://help.github.com/articles/about-pull-requests/)