Update README installation for Antigen

[gerardbosch]

Description

Until zsh-users/antigen#717 gets fixed, the
branch needs to be specified when this is not master.

Signed-off-by: Gerard Bosch [email protected]

Fixes #54

Checklist

DOES NOT APPLY FOR THIS CHANGE

• All new and existing tests pass.
• Rather than adding functions to fzf-zsh-plugin.zsh, I have created standalone scripts in bin so they can be used by non-ZSH users too.
• Any scripts added use #!/usr/bin/env interpreter instead of potentially platform-specific direct paths (#!/bin/sh is an ok exception)
• Scripts are marked executable
• Scripts do not have a language file extension unless they are meant to be sourced and not run standalone. No one should have to know if a script was written in bash, python, ruby or whatever. Not including file extensions makes it easier to rewrite the script in another language later without having to change every reference to the previous version.
• I have confirmed that the link(s) in my PR are valid.
• I have read the CONTRIBUTING document.

License Acceptance

• This repository is Apache version 2.0 licensed (some scripts may have alternate licensing inline in their code) and by making this PR, I am contributing my changes to the repository under the terms of the Apache 2 license.

[gerardbosch]

NOTE: This commit that only updates the doc, can be safely reverted if zsh-users/antigen#717 gets fixed ever.

[unixorn]

lgtm, thanks!