Add `-r,--require` documentation to README.md
Add `-r,--require` documentation to README.md

This can probably be done better, but I think it covers all aspects
of this feature.

mstade/tape,feross/tape,substack/tape,ljharb/tape,substack/tape,jtlapp/tape,feross/tape,ljharb/tape,Joris-van-der-Wel/tape,jtlapp/tape,feross/tape,substack/tape,Joris-van-der-Wel/tape,Joris-van-der-Wel/tape,ljharb/tape,mstade/tape,jtlapp/tape,mstade/tape