Revert "Temporarily revert README deletions from #13470,"

This reverts commit 54284a13c6b503d925bf6f75db2830f1af7fcf98.
Update cover.css

Moved the inset box-shadow to .site-wrapper in order to keep the shadow at the edges even if content exceeds viewport and needs scrolling.
Mozilla bug says the fix targeted v31
Merge pull request #13905 from Jimtrim/patch-1

Update cover.css
grunt
Merge pull request #13574 from twbs/reinstate-13470

Delete redundant Grunt info in README (Merge before shipping)
grunt
Merge branch 'master' of github.com:twbs/bootstrap