Add support for chai-as-promised about chai-immutable

[astorije]

Hey @jakubzitny, thanks for bringing that up!

As a matter of fact, I have used chai-immutable with chai-as-promised before. Look at this test for example. Note however that this relies on chai-as-promised being loaded after chai-immutable (or any other plugins you want to "promisify", for that matter), as shown at the beginning.

Could you confirm this answers your issue? Do you see anything we can add to chai-immutable in this regard?

[jakubzitny]

Hi, thanks for the reply :)

It really works when I load chai-as-promised at the end, great. Actually I did think about this but somehow I forgot to try loading it like this. Maybe it would help to mention it in the Installation part of README. Also some examples of chai-immutable with other plugins could be helpful.

[astorije]

Thanks @jakubzitny, I'll add a mention about loading chai-immutable before chai-as-promised in the README of this project, although I believe this has more its place in the README of chai-as-promised.

I won't come up with examples of using with other plugins as I'm not using any myself, but I'd be happy to discuss such examples here before adding them to the README :-)

[astorije]

I have added a paragraph in the doc and submitted it as a PR (#38) so that you can tell me what you think before merging it. See you there!