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~~ [*DejaVu supported*]
- [x] Burmese (Myanmar)
- [ ] ~~Tai Nüa (Myanmar)~~
- [ ] ~~Tagalog~~, ~~Hanunoo~~, ~~Buhid~~ (Philippines)
- [ ] ~~Tagbanwa (Philippines)~~
- [x] Thai
- [ ] ~~Vietnamese~~ [*DejaVu supported*]