When I generate a rails app, there is a lib/ folder underneath it. Now
what is exactly this lib/ folder for? When I put the rack library
there, it is not scanned by RoR. Actually to expand this matter, I
want to be able to put non-Rails plugin to be available for Rails and
my app (e.g including rack, arel etc). But it seems RoR does not scan
it. So what is it exactly for?