Improve the contribution guidelines

- Split it into sections
- Talk readers through the process of forking, pulling, updating etc
Merge pull request #353 from clear-y/jc/improve-contribution-guidelines

Improve the contribution guidelines
Add important parameter to edit()

Allows you to force the grid to be shown on elements
with backgrounds already set.
Merge pull request #352 from clear-y/jc/add-important-param

Add important parameter to the edit() mixin
Remove whitespace from end of CHANGELOG line
Readd cycle/uncycle to span