Document recommended Autoprefixer `browsers` setting

[cvrebert]

X-Ref: twbs/bootstrap-sass#995

If you are using Rails, autoprefixer is already set up.

Does that mean that it's included? or do we still need it as a gem but not worry about configuring it?

[glebm]

@ClaudioCarmeli It is already set up but uses the default browsers setting, which does not support some old browsers. The autoprefixer default is > 1%, last 2 versions, Firefox ESR.

The Bootstrap 4 default is different.

We should link to the Bootstrap 4 defaults and autoprefixer(-rails) docs in the documentation.