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)
Is "documentation" singular or plural?  This "they" sounds kind of strange to me in the context of "documentation", but sounds fine in the context of "docs".