add note about gulp to installation prereqs
add note about gulp to installation prereqs

bsmart/pdxr-pl,KatieMFritz/safs-patternlab-node-gulp-twig,Altinn/DesignSystem,patternfly/patternfly-atomic,pattern-lab/edition-node-gulp,patternfly/patternfly-atomic,pcvonz/pattern_lab_playground,KatieMFritz/safs-patternlab-node-gulp-twig,pattern-lab/edition-node-gulp,pcvonz/pattern_lab_playground,Altinn/DesignSystem,pcvonz/pattern_lab_playground,bsmart/pdxr-pl