Add link to zshelldoc

Author: unixorn

Diff for: README.md

@@ -93,6 +93,7 @@ Here are some useful resources to help you step up your shell scripting game:
 * [Safe Ways to do Things in Bash](https://github.com/anordal/shellharden/blob/master/how_to_do_things_safely_in_bash.md) - Excellent set of tips from the authors of [shellharden](https://github.com/anordal/shellharden)
 * [shellcheck](https://github.com/koalaman/shellcheck) is a lint for bash. It'll help you find unused variables, deprecated syntax and other things that make your bash scripts less stable. You can install it with `apt-get`, `brew`, `cabal`, or `yum`.
 * [shellharden](https://github.com/anordal/shellharden) - is a syntax highlighter and a tool to semi-automate the rewriting of scripts to ShellCheck conformance, mainly focused on quoting.
+* [zshelldoc](https://github.com/zdharma/zshelldoc) - Documentation generator for Bash & Zsh, with call-trees, comment extraction, etc.
 
 ### Powershell