Revert "Temporarily revert README deletions from #13470,"

This reverts commit 54284a13c6b503d925bf6f75db2830f1af7fcf98.
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