Adding contributing guidelines.

Includes license, workflow, and file formats sections.
Updating contributions description
Merge branch 'contributing' into master

Cherry-pick CONTRIBUTING.md from
https://github.com/swcarpentry/boot-camps.git

* contributing:
  Updating contributions description
  Adding contributing guidelines.
Move existing guidelines README -> CONTRIBUTING
Update basic contributing workflow

W. Trevor King: I changed the checkout commands to:

  $ git checkout -b <new-branch> <remote-tracking-base>

following Git's user manual [1].  This should have no effect on the
actual results, but it's more likely to show up in other examples that
the students see.

[1]: https://www.kernel.org/pub/software/scm/git/docs/user-manual.html
Clean up materials from original CONTRIBUTING
README.md: Link to CONTRIBUTING.md

On Fri, Jan 10, 2014 at 09:55:53AM -0800, Aron Ahmadia wrote:
> My only suggestion is that we keep a link from the README back to
> CONTRIBUTING, so somebody skimming/searching the front page knows
> where to go next.

On Fri, Jan 10, 2014 at 10:31:04AM -0800, Justin Kitzes wrote:
> In response to @ahmadia comment, I would suggest removing the final
> sentence "Please submit corrections or additions as pull requests,
> bug reports, or by email to admin@software-carpentry.org." from the
> opening paragraph of the README and replacing it with "To contribute
> corrections or additions to this repository, see the contributing
> guidelines." with the appropriate link.
Merge branch 'update-contributing'

Merge a slightly tweaked version of
https://github.com/swcarpentry/bc/pull/224

* update-contributing:
  README.md: Link to CONTRIBUTING.md
  Clean up materials from original CONTRIBUTING
  Update basic contributing workflow
  Move existing guidelines README -> CONTRIBUTING
  Updating contributions description
  Adding contributing guidelines.