Document how to run nebula w/ public DNS + use the Lighthouse DNS feature

[jasikpark]

From algolia search analytics, we can see that DNS is useful to be documented (plus I wanna know how it works myself)

How do hostnames turn into DNS host names?
Are the names case sensitive?
Do they support Unicode?
How do duplicate hostnames get resolved?
Can I run more than one Lighthouse DNS server?

[jasikpark]

slackhq/nebula#110