don't hook on postinstall, current dev cycle uses (abuses) npm i to build

so no point making it take longer than it has to for the devs only testing
webapp

build:electron - hook npm run install:electron
install:electron - run npm i for the electron deps
electron - start the app locally for testing

add to README

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>