Update README.md / Docs

[madhavajay]

Can we do this for:

• covidwatch-android
• covidwatch-android-minimal
• tcn-client-android

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

[madhavajay]

There are a tonne of badges that can show us what languages / frameworks we use, what the current test results are on CI, what version is currently available etc, which I think all add nice readability.

[nkumarcc]

This is done for this repo and minimal. @Apisov can you do for tcn-client? Honestly have never run or set that one up by itself