di: Add more docs and examples

[k0001]

Particularly, explain why people should perhaps consider looking at di-core and stuff like that.

[chshersh]

I also would like to see project examples which use di logging framework. I came up with almost same approach independently in one logging library. Looks like this way of logging is reasonable to many people... So I would like to see how di is actually used and what is users feedback. Thanks in advance!

[k0001]

@ChShersh I don't know of projects using di yet other than my private own. To be fair, though, this makes sense to some extent, seeing how the most recent version of di (which changed radically from the one before, which I considered insufficient) was released very recently, and how little advertisement I have made. @cocreature mentioned some time ago that he was using di (more likely, di-core nowadays), but I don't know if he still is.

Seeing as most projects that do logging tend to be private, it would be nice to have a list (maybe in the README) where people using di could mention their projects if they wanted to. Interestingly, though, di (and more generally, di-core) makes it rather unobtrusive for libraries themselves to do logging by simply requiring that somebody provides a Di at some point (technically, MonadDi works as well, but that can be noisy in a library and not different from the dozen of mtl-like approaches to logging out there). I'm currently working on a migrations library that uses Di in this way.