Use Droid Fallback as a fallback before Unifont

Feedback is that Droid Fallback looks better than Unifont. As Droid
Fallback doesn't have bold, hopefully we don't get this far, but
it beats Unifont.

The droid fonts can be found in the fonts-droid package on Ubuntu.

A readme section to list required font packages has also been added
Add Arundina Sans to font list

Arundina Sans is based on DejaVu and has bold and italic versions, so it's an easy choice. Obtained with the ``fonts-sipa-arundina`` package.

This change makes mapnik/mapnik#2161 more obvious, but we were probably encountering that bug before with Unifont, it's just hard to tell with how hard Unifont was to read.
Add Padauk to font list

Padauk has both a normal and a bold version, making it a good choice.

It's also the only packaged Burmese font I could find.
Add text explaining what happens with missing fonts and how only fonts for languages you care about are required
Add Khmer OS Metal Chrieng Regular to font list

It's been recommended by a native Khmer speaker, and is packaged
on Ubuntu.

Also split the font lists into multiple lines because it's getting long