Minor updates and fixes to the CONTRIBUTING doc

* Fixed some uses of docker v. Docker
* Formatting and line wrapping.
* Spelling errors and grammar fixes.

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
on both client and server (if running using a remote Docker daemon, or boot2docker - I'll put in a PR to add arch to `docker version`)