Add stub-prime and stub-first to README

[JeremyRand]

stub-prime and stub-first should be documented in the README, similar to https://github.com/namecoin/ncdns-nsis/blob/master/ncdns-inst.conf.in

[hlandau]

These default to no anyway, so explicitly listing them is unnecessary.

[JeremyRand]

Given that these options are more critical for our use case than for most Unbound use cases, I think it's safer to include them explicitly (in case Unbound changes their defaults). Also we should be consistent between the ncdns docs and what ncdns-nsis does.