[risk=no] Add UI setup steps to README (#2044)
[risk=no] Add UI setup steps to README (#2044)

* Need a recent Node.js

* add yarn install step

all-of-us/workbench,all-of-us/workbench,all-of-us/workbench,all-of-us/workbench,all-of-us/workbench,all-of-us/workbench,all-of-us/workbench,all-of-us/workbench,all-of-us/workbench