Update ntr.md

Just proposing some tightening up of the Terms & Fees wording:

Clarify that fee is not due if candidate already known

Refund should be due if employee leaves within 12 weeks of starting work, not 12 weeks of introduction?

Clarify that replacement candidate must be acceptable, can't avoid refund by simply finding any candidate
Merge pull request #4 from brucegreig/patch-1

Update ntr.md