Draft a documentation about the continuous integration

docs/continuous_integration.md

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+# Continuous integration strategy
+
+<!--- TOC -->
+
+* [Introduction](#introduction)
+* [CI tools](#ci-tools)
+* [Rules](#rules)
+* [What is the CI checking](#what-is-the-ci-checking)
+* [What is the CI reporting](#what-is-the-ci-reporting)
+* [Current choices](#current-choices)
+  * [R8 task](#r8-task)
+  * [Android test (connected test)](#android-test-connected-test)
+
+<!--- END -->
+
+## Introduction
+
+This document gives some information about how we take advantage of the continuous integration (CI).
+
+## CI tools
+
+We use GitHub Actions to configure and perform the CI.
+
+## Rules
+
+We want:
+
+1. The CI to detect as soon as possible any issue in the code
+2. The CI to be fast - it's run on all the Pull Requests, and developers do not like to wait too long
+3. The CI to be reliable - it should not fail randomly
+4. The CI to generate artifacts which can be used by the team and the community
+5. The CI to generate useful logs and reports, not too verbose, not too short
+6. The developer to be able to run the CI locally - to help with this we have [a script](../tools/check/check_code_quality.sh) the can be run locally and which does more checks that just building and deploying the app.
+7. The CI to be used as a common environment for the team: generate the screenshots image for the screenshot test, build the release build (unsigned)
+8. The CI to run repeated tasks, like building the nightly builds, integrating data from external tools (translations, etc.)
+9. The CI to upgrade our dependencies (Renovate)
+10. The CI to do some issue triaging
+
+## What is the CI checking
+
+The CI checks that:
+
+1. The code is compiling, without any warnings, for all the app build types and variants and for the minimal app
+2. The tests are passing
+3. The code quality is good (detekt, ktlint, lint)
+4. The code is running and smoke tests are passing (maestro)
+5. The PullRequest itself is good (with danger)
+6. Files that must be added with git-lfs are added with git-lfs
+
+## What is the CI reporting
+
+The CI reports:
+
+1. Code coverage reports
+2. Sonar reports
+
+## Current choices
+
+### R8 task
+
+The CI does not run R8 because it's too slow, and it breaks rule 2.
+
+The drawback is that the nightly build can fail, as well as the release build.
+
+Since the nightly build is failing, the team can detect the failure quite fast and react to it.
+
+### Android test (connected test)
+
+We limit the number of connected tests (tests under folder `androidTest`), because it often break rule 2 and 3.