Update OnetoOne.md

[Noitidart]

Add additional explanation on the automatic one-to-one relationship.

[sailsbot]

Hi @Noitidart! It looks like the title of your pull request doesn’t quite match our guidelines yet. Would you please edit your pull request's title so that it begins with [proposal], [patch], [fixes #<issue number>], [implements #<other PR number>], or [misc]? Once you've edited it, I'll take another look!