Give example on how to filter out diacritics

[jonathan-s]

Fix #84. Since there are a few people requesting this, myself included when I tried sorting this out, I'm adding this to the documentation as an example.

This also showcases that strings are in fact decomposed in the iterator, which the initial example does not.

[Manishearth]

I don't think this is an appropriate use of this crate: this is narrowly stripping Latin diacritics, nothing else (and that too, only for Latin letters in ASCII).