Add DS_Store to .gitignore and add a Contributing.md

`DS_Store` shouldn't be committed to git.

Would be handy to have a `contributing.md` so that new members of the team know how they can contribute (e.g. to update documentation or fix bugs etc) and same basic team rules :smile: 

Could also consider adding an [issue and/or pull request templates](https://github.com/blog/2111-issue-and-pull-request-templates) :smile_cat: 
