Enhance the "getting started" docs a little more

[PeteDuncanson]

Describe the bug

Seems the readme is missing some of the usual simple things I would expect for getting started, they can but found but it makes you have to go look for them.

How to install

I think these are both right but feel free to check/change:
`
npm install xero-node -D

yarn add xero-node
`

Include require statements in all demo code

Can we include the right import/require statements somewhere, had to do some guess work to find it:

const { XeroClient } = require("xero-node");

This won't work with SPA as we don't support it

I tried to use this in my SPA but its using loads of node only goodies so it failed. I then used it to as a crib sheet to try to make my own thin client to get it working (I only needed two API endpoints). But this fell down due to CORS errors/restrictions when got to actually trying to do the API calls (the OAuth stuff doesn't have this problem so lead me on a bit of a false start). Would really be worth a bit about the lack of CORS support and how a proxy or Lambda would be needed to do the heavy lifting instead. Would have saved me a day of swearing to know that up front.

All the above small bits of info would have saved me some time and head scratching, less friction so I can get to the good stuff quicker.

Cheers

Pete

[SerKnight]

Note to also add AuthEventId and the logic of updateTenants to readme

#475