Add an option to README for dev tokens to be at `CUSTOMER.quip.com`

[benrucker]

I recently wanted to find out if I could use this plugin and ended up thinking that I don't have access to the Quip API because the table of API hostnames led me to a "Not found" page, despite following it correctly for my deployment.

Turns out my link was not at CUSTOMER.onquip.com but just CUSTOMER.quip.com. This seems worth adding to the readme, though I don't have enough time to add it myself at the current moment.