Document Git symbol require powerline patched font
Document Git symbol require powerline patched font

Close #92

See the related discussion at
https://github.com/denysdovhan/spaceship-zsh-theme/pull/78#discussion_r115125008

bytefluxio/spaceship-zsh-theme,denysdovhan/spaceship-zsh-theme,nelsonghezzi/spaceship-zsh-theme