docs: what the CI does (elastic#1413)

.github/workflows/README.md

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+## CI/CD
+
+There are 5 main stages that run on GitHub actions:
+
+* Linting
+* Test
+* Test `main` versions
+* Package
+* Release
+
+There are some other stages that run for every push on the main branches:
+
+* [Microbenchmark](./microbenchmark.yml)
+* [Snapshoty](./snapshoty.yml)
+
+### Scenarios
+
+* Compatibility matrix runs on branches, tags and PRs basis.
+* 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.
+* Automated release in the CI gets triggered when a tag release is created.
+* **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.
+
+### Compatibility matrix
+
+Ruby agent supports compatibility to different ruby versions and frameworks, those are defined in:
+
+* [frameworks](../../.ci/.framework.yml) for all the PRs.
+* [frameworks](../../.ci/.main_framework.yml) for all the PRs using the `main` branches.
+* Ruby [versions](../../.ci/.ruby.yml) for all the `*nix` builds.
+* [Exclude list](../../.ci/.exclude.yml) for the above entries.
+
+### 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 whole workflow will compile and test every entry in the compatibility matrix for Linux.
+
+### Release process
+
+This process has been fully automated and it gets triggered when a tag release has been created.
+The tag release follows the naming convention: `v.<major>.<minor>.<patch>`, where `<major>`, `<minor>` and `<patch>`.
+
+### OpenTelemetry
+
+There is a GitHub workflow 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).
+
+## Bump automation
+
+[updatecli](https://www.updatecli.io/) is the tool we use to automatically update the specs
+the [APM agents](./updatecli.yml) use.