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)
Update README.md

Docker-DCO-1.1-Signed-off-by: Hyeongkyu Lee <hyeongkyu.lee@navercorp.com> (github: leeplay)
We haven't required lxc and aufs for years now...

Well, maybe not years, but internet years...

Docker-DCO-1.1-Signed-off-by: Timothy Hobbs <timothyhobbs@seznam.cz> (github: timthelion)
Update contrib/mkimage/debootstrap to add "sqeeze-lts" to "oldstable" as well

Note that the archive does _not_ have a symlink from oldstable-lts to squeeze-lts, hence the change from using "${suite}" to not using it in here and just using the string "squeeze-lts" directly regardless of whether we're building a "squeeze" image or an "oldstable" image.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6858 from timthelion/patch-6

We haven't required lxc and aufs for years now...
Merge pull request #6849 from leeplay/patch-1

Update README.md
Merge pull request #6760 from jamtur01/contrib

Minor updates and fixes to the CONTRIBUTING doc
understanding-docker: Use consistent term

Term 'service' was used together with 'daemon'. It would be better to use the term 'daemon' solely because most part of documentation are using the term.
Merge pull request #6899 from sjp38/patch-1

understanding-docker: Use consistent term
Merge pull request #6897 from tianon/moar-squeeze-lts

Update contrib/mkimage/debootstrap to add "sqeeze-lts" to "oldstable" as well