Document ncdt and ncdumpzone

[JeremyRand]

ncdt and ncdumpzone are undocumented at the moment. See chatlog below:

Jeremy_Rand hl: What does ncdt do? I've never used it, and I can't tell from glancing at the source code what its purpose is

hl Jeremy_Rand: It outputs BIND zone file formatted records for a Namecoin d/ name and JSON value provided on the command line J Jeremy_Rand hl: how does this differ from ncdumpzone?

hl Jeremy_Rand: that just outputs a BIND zonefile for the entirety of .bit