Update README for git-prompt to clarify how to use (#10922)

* Add note about need to customise theme.

It may not be obvious that the theme needs to be customised. See: https://github.com/ohmyzsh/ohmyzsh/issues/9395

* Clarify type of theme customisation needed.

plugins/git-prompt/README.md

@@ -9,6 +9,10 @@ To use it, add `git-prompt` to the plugins array in your zshrc file:
 plugins=(... git-prompt)
 ```
 
+You may also need to [customize your theme](https://github.com/ohmyzsh/ohmyzsh/issues/9395#issuecomment-1027130429) 
+to change the way the prompt is built. See the
+[OMZ wiki on customizing themes](https://github.com/ohmyzsh/ohmyzsh/wiki/Customization#overriding-and-adding-themes).
+
 See the [original repository](https://github.com/olivierverdier/zsh-git-prompt).
 
 ## Requirements