Add Southeast Asia fonts

**Work in progress, do not merge yet**

Based on issues raised in #294, but I'm not yet tackling East Asia and CJK fonts with the Han Unification issues.

See the [Unicode Font Guide](http://www.unifont.org/fontguide/) for what I am considering S.E. Asia

- [x] Add readme section
- [x] Add Droid Sans as penultimate fallback
- [ ] Khmer
- [ ] Lao
- [ ] Burmese (Myanmar)
- [ ] Tai Nüa (Myanmar)
- [ ] Tagalog, Hanunoo, Buhid, Tagbanwa (Philippines)
- [ ] Tagbanwa (Philippines)
- [ ] Thai
- [ ] Vietnamese [*Already supported?*]