Update non_null.ex with details about adding and removing

[alanvardy]

Hello and thank you for the project.

I was confused around adding and removing non_null now that the correctness is more strictly enforced with 1.7.0. I received clarification from @benwilson512 and hope that this edit helps. I can reword this if anyone can think of a better way of putting it.

[benwilson512]

Thank you!

[maartenvanvliet]

There's a bit of nuance here.

Adding non_null/1 to an input type is a breaking change, whereas adding non_null/1 to an output type is not.
The inverse also holds, removing non_null from an input type is non-breaking, removing non_null/1 from an output type is breaking.

[benwilson512]

@maartenvanvliet ah yes good point, I was coming at it from the angle of inputs given the recent bug fix. I'll make an update to handle both situations.

[alanvardy]

Very interesting, thank you @maartenvanvliet and @benwilson512 !