Add antigen compatible plugin description

[matthewfranglen]

This adds antigen support for ZSH. See https://github.com/zsh-users/antigen for more details.

This allows a user to start using this library using the command:

antigen bundle matthewfranglen/gitflow-avh --branch=master

Which handles downloading the repository and adding the command to your path.

[matthewfranglen]

If you choose to use the master branch then the gitflow-avh.plugin.zsh file must be present on that branch. If it is missing then the git flow commands will not be added to the path.

[ChrisJStone]

I'm in the process of updating gitflow. If you could kindly resubmit your pull request against my fork here as it appears this fork is no longer being updated.