autojump: clarify need to install it first

Related: #8806

plugins/autojump/README.md

@@ -8,4 +8,4 @@ To use it, add `autojump` to the plugins array in your zshrc file:
 plugins=(... autojump)
 ```
 
-More info on the usage: https://github.com/wting/autojump
+**Note:** you have to [install autojump](https://github.com/wting/autojump#installation) first.

plugins/autojump/autojump.plugin.zsh

@@ -29,6 +29,6 @@ if (( ! found && $+commands[brew] )); then
   fi
 fi
 
-(( ! found )) && echo '[oh-my-zsh] autojump script not found'
+(( ! found )) && echo '[oh-my-zsh] autojump not found. Please install it first.'
 
 unset autojump_paths file found