Update README.md to include CI documentation

[AndreSand]

Thanks for submitting a pull request. Please include the following information.

What I have done and why
Add CI documentation to readme

Include a summary of what your pull request contains, and why you have made these changes.

Fixes #<issue_number_goes_here>

How I'm testing it

Choose at least one:

• Unit tests
• UI tests
• Screenshot tests
• N/A (provide justification) Only adding documentation

Do tests pass?

• Run local tests on DemoDebug variant: ./gradlew testDemoDebug
• Check formatting: ./gradlew --init-script gradle/init.gradle.kts spotlessApply

Is this your first pull request?

• Sign the CLA
• Run ./tools/setup.sh
• Import the code formatting style as explained in the setup script.

[JackEblan]

Can you also add a documentation for how they implement code coverage properly🙏

[JoseAlcerreca]

We'll add this to https://github.com/android/nowinandroid/wiki/Testing-strategy-and-how-to-test once it's moved back to /docs