docs: Document the option to bump build number
docs: Document the option to bump build number

Running `grunt bump:build` increments the build number rather than one of the major, minor, and patch version fields.
XciD/grunt-bump,halkon/grunt-bump,aaccurso/grunt-bump-cordova,walmartlabs/grunt-bump,dan-kushnaryov/grunt-bump,vojtajina/grunt-bump,blargism/grunt-bump,legalthings/grunt-bump,eddiemonge/grunt-bump,tischler/grunt-bump