Implement man page for mkdir, print it when executing mkdir --help.

Created by: GedRap

Hey,

My first attempt to contribute to this project! Saw the issue #19 (closed), thought it's a good starting point.

Didn't notice a consistent pattern for printing man pages (e.g. test prints with --help, du is not printing it but has #![deny(warnings)] comment out). Not sure if I missed one? So decided that --help option seems reasonable. Totally happy to change it if I missed some convention!

The build is passing here https://travis-ci.org/GedRap/coreutils/builds/130974461 .

Thanks.