Update contributing guidelines
Update README
Updated polish translation
Remove WP Super Cache, add Soil to Recommended Plugins

Recommend only W3TC instead of both W3TC and Super Cache since it
could be confusing:

https://groups.google.com/forum/?fromgroups=#!topic/roots-theme/nOfLGhq90Ow
Merge pull request #681 from netboy/master

Update Polish translation
root update docs