docs: add code coverage documentation

Author: wzieba

docs/code-coverage.md

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+# Code coverage
+
+This project uses [Kover](https://github.com/Kotlin/kotlinx-kover) tool for generating code coverage metrics.
+To run the code coverage report for the whole codebase, run `./gradlew koverHtmlReport` and open the HTML report at `WordPress/build/reports/kover/html/index.html`.
+
+## Coverage exclusions
+
+To get more precise results of the metrics, some classes are excluded from code coverage calculations. Those classes are e.g. Dagger/Hilt generated code, Acitivies, Fragments, databinding etc. A complete list of exclusions is available in the `config/gradle/code_coverage.gradle` file.
+
+## Codecov
+
+We also have `Codecov` integration for getting reports on each PR and observing trends. See [the dashboard](https://app.codecov.io/github/wordpress-mobile/WordPress-Android/) to get more insights.