Update README: Improve `provide` config docs.
Update README: Improve `provide` config docs.

Re: #436, #383

greypants/gulp-starter,jakefleming/mezzotent,vigetlabs/gulp-starter,jakefleming/mezzotent,greypants/gulp-starter,greypants/gulp-starter,greypants/gulp-starter,vigetlabs/gulp-starter,jakefleming/mezzotent,vigetlabs/gulp-starter,vigetlabs/gulp-starter,jakefleming/mezzotent