Document recommended Autoprefixer `browsers` setting

Created by: cvrebert

The docs mention that Autoprefixer is required, but don't cover what browsers setting is required for parity with precompiled Bootstrap. To avoid having two copies which are liable to get out-of-sync, I propose moving the data out of Gruntfile.js and into a separate file. The docs can then simply say to consult the file and link to it. X-Ref: https://github.com/twbs/bootstrap-sass/pull/995