Update README.md

[madhavajay]

Can we do this for:

covidwatch-ios
covidwatch-ios-minimal
tcn-client-ios
We want the following out of our README.md

How to Setup

This is the "Onboarding from scratch" for anyone new to read and should work
Explanation of what it does

How it interfaces with other COVIDWatch / external repos

How testing works

How to Contribute

Style guides etc
Badges
Lets add some cool badges:
https://gist.github.com/tterb/982ae14a9307b80117dbf49f624ce0e8

FAQ
When people are unsure lets populate this with some obvious answers

Contributors / Contacts

Link to the COVIDWatch Slack or how to email to get access / invites
@name's of people you can contact for help regarding the project.

Heres an example of some stuff I did in the cloud functions readme:

https://github.com/covid19risk/covidwatch-cloud-functions