Clarify that other distrinbuted version control systems exist

In particular make the claim specific to distributed version control
systems, rather than specific to the tool Git.
Merge pull request #38 from garethr/distributed_versioning_clarification

Clarify that other distrinbuted version control systems exist
Fix broken link to Git
Merge pull request #46 from johncowie/master

Removing an apostrophe.
Add a suggested CONTRIBUTING.md file
Merge pull request #47 from alphagov/add-contribution-guidelines

Add a suggested CONTRIBUTING.md file
Add line about proofreading and amends

Remove punctuation from end of bullet points as per the styleguide
Remove repetition, fix typo
Removed superfluous audience
added reference to WhiteHouse standards github project
Fix rendering of bullets
missing space in Person spec added for bullets
sense change over sprint length
Expand contribution guidance
missing from to
Amends based on feedback from @mookin
Merge pull request #48 from alphagov/contribution-guide

Expand contribution guidance
Fix typo (x2)
Add breadcrumbs for pages where they were missing
Change feedback link to main GOV.UK feedback form