powerline.elv: optional fonts package needs to be installed when using Ubuntu

[rubiojr]

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I found that some optional fonts need to be installed for the powerline prompt to work. See the prompt without the fonts-powerline package installed:

Other distros may not need this or the package name could be different though 🤷‍♂️.

@muesli: would you like a note about this to be added to the header docs here or the README?

[muesli]

Interesting, I don't have any powerline fonts installed on my Arch Linux, so I assumed I just picked more common glyphs.

Guess I was wrong. In that case we should definitely document it!

[dcu]

having the same issue, what would be the font to install? the glyphs used by chain work

[rubiojr]

@dcu in Ubuntu (and Debian I assume) it's the fonts-powerline package.