issue#176: document `yarn add expo` for devs

[jzacsh]

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

1. [x] call out yarn add expo that tasn explained
2. [x] 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.