Quickstart doc doesn't define WithNullable

[FranklinChen]

quickstart.md doesn't define WithNullable. I searched the repo and found a definition of it in QuickStart.purs. Maybe there should be a script to insert the code into the doc, to keep them in sync?

[justinwoo]

There are rarely edits made nowadays to the file, so it'll just have to be copied over.