Update README.md

I guess in most cases zlib is installed by default, but in case it is not, build would failed as digest depends on it.

Author: Shenmin-Z

README.md

@@ -168,7 +168,7 @@ sudo apt install libicu-dev libtinfo-dev libgmp-dev
 **Fedora**:
 
 ```bash
-sudo dnf install libicu-devel ncurses-devel
+sudo dnf install libicu-devel ncurses-devel # also zlib-devel if not already installed
 ```
 
 #### Windows-specific pre-requirements