Document the functionality of the placeholder operator character `_`

[cipherself]

Currently there isn't documentation explaining how the _ character works

[barca]

Are you referring to its usage or implementation. Currently the book provides a very brief description of what _ does, however I could write up something slightly more detailed if desired. (see below):
http://blog.vrinek.io/rust/doc/book/patterns.html

[cipherself]

@barca Hmm, the conversation that starts here https://botbot.me/mozilla/rust/2015-05-25/?msg=40115096&page=31 might be interesting