Add test framework architecture diagram. (#3999)

Also link the test framework developer guide from the main readme.

README.md

@@ -18,6 +18,11 @@ The source files for the general Elastic Agent documentation are currently store
 in the [ingest-docs](https://github.com/elastic/ingest-docs/tree/main/docs/en/ingest-management) repository.
 The following docs are only focused on getting developers started building code for Elastic Agent.
 
+### Test Framework
+
+In addition to standard Go tests, changes to the Elastic Agent are always installed and tested on cross-platform virtual machines.
+For details on writing and running tests see the [Test Framework Developer Guide](docs/test-framework-dev-guide.md).
+
 ### Changelog
 
 The changelog for the Elastic Agent is generated and maintained using the [elastic-agent-changelog-tool](https://github.com/elastic/elastic-agent-changelog-tool). Read the [installation](https://github.com/elastic/elastic-agent-changelog-tool/blob/main/docs/install.md)

docs/test-framework-dev-guide.md

@@ -1,5 +1,13 @@
 # Developer Guide for the Integration and E2E Testing Framework
 
+The agent testing framework allows running integration and end-to-end tests against real agents
+installed on remotely provisioned virtual machines. The current set of integration tests can be
+found in https://github.com/elastic/elastic-agent/tree/main/testing/integration. Each test must
+start with a `define.Require` declaration describing how the test can be run. The diagram below
+provides a high level overview of the testing framework.
+
+<center><img src="./images/testing-framework.png" alt="Elastic Agent Testing Framework"></center>
+
 ## Prerequisites
 
 ### Dependencies