add git completion requirements notice

plugins/git-flow-avh/README.md

@@ -3,10 +3,17 @@
 This plugin adds completion for the [git-flow (AVH Edition)](https://github.com/petervanderdoes/gitflow-avh).
 The AVH Edition of the git extensions that provides high-level repository operations for [Vincent Driessen's branching model](https://nvie.com/posts/a-successful-git-branching-model/).
 
-The git-flow tool has to be [installed](https://github.com/petervanderdoes/gitflow-avh#installing-git-flow) separately.
-
 To use it, add `git-flow-avh` to the plugins array in your zshrc file:
 
 ```zsh
 plugins=(... git-flow-avh)
 ```
+
+## Requirements
+
+1. The git-flow tool has to be [installed](https://github.com/petervanderdoes/gitflow-avh#installing-git-flow)
+   separately.
+
+2. You have to use zsh's git completion instead of the git project's git completion. This is typically
+   done by default so you don't need to do anything else. If you installed git with Homebrew you
+   might have to uninstall the git completion it's bundled with.