added note for mutually_exclusive params

[gencer]

No description provided.

[dblock]

This is related to #1717. I think this note makes it even more confusing, what should happen is that if you specify a default for more than one parameter (or any parameter) in a mutually exclusive relationship you should get an error on mounting the API.

[gencer]

But, I am sure people will fall into this if they simply forget :default in their optional params when they are using exactly_one_of. To prevent this, and let them know that those params should not exists even as a nil, we should add a note/warning.

Still, It is your decision of course :)

[dblock]

I just think the language of the note is a bit confusing and unnecessarily alarming :) Maybe make it consistent with other notes. Maybe something like this?

Note that using :default with mutually_exclusive will cause multiple parameters to always have a default value and raise a Grape::Exceptions::Validation mutually exclusive exception.

No need to CHANGELOG README changes either, you can remove that. Amend, squash and I'll merge?

[grape-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#1720](https://github.com/ruby-grape/grape/pull/1720): Added note for avoid using :default on exactly_one_of - [@gencer](https://github.com/gencer).

Generated by 🚫 danger

[gencer]

And you think the right thing. I agree on that.

I amend and squash changes (removed changelog but change the readme)

[dblock]

Thanks, merged.