Document better the package-and-qa job

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/testing_guide/end_to_end_tests.md

@@ -17,11 +17,13 @@ a black-box testing framework for the API and the UI.
 
 We run scheduled pipeline each night to test nightly builds created by Omnibus.
 You can find these nightly pipelines at [gitlab-org/quality/nightly/pipelines][quality-nightly-pipelines].
+Results are reported in the `#qa-nightly` Slack channel.
 
 ### Testing staging
 
 We run scheduled pipeline each night to test staging.
 You can find these nightly pipelines at [gitlab-org/quality/staging/pipelines][quality-staging-pipelines].
+Results are reported in the `#qa-staging` Slack channel.
 
 ### Testing code in merge requests
 
@@ -40,6 +42,29 @@ Below you can read more about how to use it and how does it work.
 Currently, we are using _multi-project pipeline_-like approach to run QA
 pipelines.
 
+![QA on merge requests CI/CD architecture](img/qa_on_merge_requests_cicd_architecture.png)
+
+<details>
+<summary>Show mermaid source</summary>
+<pre>
+graph LR
+    A1 -.->|1. Triggers an omnibus-gitlab pipeline and wait for it to be done| A2
+    B2[<b>`Trigger-qa` stage</b><br />`Trigger:qa-test` job] -.->|2. Triggers a gitlab-qa pipeline and wait for it to be done| A3
+
+subgraph gitlab-ce/ee pipeline
+    A1[<b>`test` stage</b><br />`package-and-qa` job]
+    end
+
+subgraph omnibus-gitlab pipeline
+    A2[<b>`Trigger-docker` stage</b></b><br />`Trigger:gitlab-docker` job] -->|once done| B2
+    end
+
+subgraph gitlab-qa pipeline
+    A3>QA jobs run] -.->|3. Reports back the pipeline result to the `package-and-qa` job<br />and post the result  on the original commit tested| A1
+    end
+</pre>
+</details>
+
 1. Developer triggers a manual action, that can be found in CE / EE merge
    requests. This starts a chain of pipelines in multiple projects.