This could get you started: http://programmingthomas.com/blog/2015/1/27/nim-on-ios
And this could get you stopped: https://gradha.github.io/articles/2014/03/nimrod-for-cross-platform-software.html
I don't know how accurate all of this is today!
I think Yuriy succeeded in writing a small cross-platform framework that works on desktop, android, iOS and even JavaScript: https://github.com/yglukhov/nimx
I haven't looked at it much yet, but it seems like a good start.
The pain points that are not explained yet are the infrastructure related matters, i.e. what SDKs and software needs to be installed, how to deploy compiled programs to the device and, most importantly, how to debug the runtime. Someone should write a tutorial - preferably before I have time to work on cross-platform projects myself wink wink