docs: explain what the CI does (#11464)

(cherry picked from commit e4da05a)

.github/workflows/README.md

@@ -0,0 +1,53 @@
+# CI/CD
+
+There are 3 main stages that run on GitHub actions for every push or Pull Request:
+
+* Linting
+* Test
+* System tests
+
+There are some other stages that run for every push on the main branches:
+
+* [Microbenchmark](./microbenchmark.yml)
+* [Benchmarks](./benchmarks.yml)
+* Packaging - see below.
+
+## Scenarios
+
+* Tests should be triggered on branch, tag and PR basis.
+* Commits that are only affecting the docs files should not trigger any test or similar stages that are not required.
+* **This is not the case yet**, but if Github secrets are required then Pull Requests from forked repositories won't run any build accessing those secrets. If needed, then create a feature branch.
+
+## How to interact with the CI?
+
+### On a PR basis
+
+Once a PR has been opened then there are two different ways you can trigger builds in the CI:
+
+1. Commit based.
+1. UI based, any Elasticians can force a build through the GitHub UI
+
+### Branches
+
+Every time there is a merge to main or any release branches the main workflow will lint and test all for Linux, Windows and MacOS.
+
+## Package
+
+The packaging and release automation relies on the Unified Release process and Buildkite for generating the
+`DRA` packages, for further details please go to [the buildkite folder](../../.buildkite/README.md).
+
+## OpenTelemetry
+
+There is a [GitHub workflow](./opentelemetry.yml) in charge to populate what the workflow run in terms of jobs and steps. Those details can be seen in
+[here](https://ela.st/oblt-ci-cd-stats) (**NOTE**: only available for Elasticians).
+
+## Smoke tests
+
+Smoke tests are triggered based on a specific scheduler, see [smoke-tests-ess](./smoke-tests-ess.yml) and [smoke-tests-os](./smoke-tests-os.yml)
+for further details.
+
+## Bump automation
+
+[updatecli](https://www.updatecli.io/) is the tool we use to automatically bump some of the dependencies related to
+the [Elastic Stack versions](./bump-elastic-stack.yml) used for the different tests, the `elastic/Beats.git`
+[specific version](./bump-elastic-stack.yml) used by the APM Server or the [Golang bump](./bump-golang.yml) automation.

README.md

@@ -1,5 +1,6 @@
-[![Build Status](https://apm-ci.elastic.co/buildStatus/icon?job=apm-server/apm-server-mbp/main)](https://apm-ci.elastic.co/job/apm-server/job/apm-server-mbp/view/change-requests/job/main/)
+[![ci](https://github.com/elastic/apm-server/actions/workflows/ci.yml/badge.svg)](https://github.com/elastic/apm-server/actions/workflows/ci.yml)
 [![Smoke Tests](https://github.com/elastic/apm-server/actions/workflows/smoke-tests-schedule.yml/badge.svg)](https://github.com/elastic/apm-server/actions/workflows/smoke-tests-schedule.yml)
+[![Package status](https://badge.buildkite.com/fc4aa824ffecf245db871971507275aa3c35904e380fef449c.svg?branch=main)](https://buildkite.com/elastic/apm-server-package)
 
 # APM Server