zsh completion instructions are hardcoded

https://github.com/gradle/gradle-completion/blob/master/README.md

```
# Ensure /usr/local/share/zsh/site-functions is on $fpath. You should get a result from the following
echo $fpath | grep "/usr/local/share/zsh/site-functions"
```

May not be correct on some setups.