Add CONTRIBUTING.md file

Most HTML markup is stripped out, however there are a few exceptions.
I'm not sure it's really a great idea to put funky CSS in a markdown document.
Might be better idea to just go pure markdown?
Remove remaining HTML from CONTRIBUTING.md
Merge pull request #69 from caitp/issue-68

Fixed #68: Add CONTRIBUTING.md file
Merge branch 'next'
updates download zip
Merge branch 'master' of github.com:jlukic/Semantic-UI
fixes popup example
fixes name conflict with url icon and link variation. formatting on popup module
fixes text item example
fixes popup icon in message
fixes link namespace collision with link icon
Updates with some new google pasta images, updates list view examples,
adds relaxed list