issue#176: document yarn add expo for devs

[jzacsh]

(WIP) things I think would be helpful to cover:

1. call out yarn add expo that tasn explained
2. replace probably(?)-copy-pasta repo URL in the instructoins with etesync org
3. should probably actually explain that yarn add expo's corresponding
   package.json,yarn.lock changes should be ignored and not accidentally
   committed.
4. explain if/should you have started a etebase backend (and if so, link to the right repo's instructions)
5. should explain how to setup a fake account/backend (so you're not local-testing against your personal prod data)

[tasn]

I think the last two are probably not needed in detail. But rather have a mention about it and point to the server repo for how to do it.