Documentation needed regarding nyc config key priority

[r4j4h]

Different sources indicate that the nyc config key can be read from either package.json or .babelrc.

In the case that it is defined in both, which takes priority?

This derived from a side conversation from issue #15 and was created so it can remain on track.

[bcoe]

@r4j4h sorry for not following up sooner; would love help adding this documentation if you have any interest in contributing 😄