Update antigen install instructions.
Update antigen install instructions.

Antigen doesn't cope when the default branch of a repo is not named
`master` - see https://github.com/zsh-users/antigen/issues/717

Update the install instructions in the readme accordingly.

Signed-off-by: Joe Block <13bb081c6ac072ef78a3e348068e2204400d7335@unixorn.net>

unixorn/git-extra-commands,unixorn/git-extra-commands,unixorn/git-extra-commands,unixorn/git-extra-commands