Add highlighter triggered when the user is root

highlighters/root/README.md

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+zsh-syntax-highlighting / highlighters / root
+=================================================
+
+This is the ***root*** highlighter, that highlights the whole line if the current user is root.
+
+
+How to activate it
+------------------
+To activate it, add it to `ZSH_HIGHLIGHT_HIGHLIGHTERS`:
+
+    ZSH_HIGHLIGHT_HIGHLIGHTERS=( [...] root)
+
+
+How to tweak it
+---------------
+This highlighter defines the following styles:
+
+* `root` - the style for the whole line if the current user is root.
+
+To override one of those styles, change its entry in `ZSH_HIGHLIGHT_STYLES`, for example in `~/.zshrc`:
+
+    ZSH_HIGHLIGHT_STYLES[root]='bg=red'
+
+The syntax for declaring styles is [documented here](http://zsh.sourceforge.net/Doc/Release/Zsh-Line-Editor.html#SEC135).