Update README.md

[clwy-cn]

Solved the problem in https://github.com/twbs/bootstrap-rubygem/issues/248

[tgaff]

I also had to do this to get JS working.

I would suggest making it clear whether or not the sprockets and import maps sections are "AND" or "OR". If they're intended to be "and" then this README update could potentially confuse the situation. Following on from import maps to the sprockets section accomplishes what this README change suggests (though, the //= require bootstrap-sprockets portion likely doesn't work.)

I think they're intended as OR and that could be made clearer.

[tgaff]

Also link #246

[Seven-dev7]

Is it possible to merge this PR, those informations insides are very USEFUL, thank U Guys! @twbs @glebm @thomas-mcdonald