docs: add information about scripted input tests images (#181)

This PR updates Development flow section of README file. It adds
information about scripted input tests images creation.

README.md

@@ -20,3 +20,4 @@ Once new version of Splunk or sc4s is released and PR with updates in SC4S_matri
 3. In this PR, modify the matrix step to reference the branch of `addonfactory-test-matrix-action` that is currently under test.
 4. Execute CI for several TAs with `build-test-release` workflow referencing created branch on `addonfactory-workflow-addon-release`
 5. After succesfull execution of tests, make a new fix release of `addonfactory-test-matrix-action` which will be automatically incorporated into latest `addonfactory-workflow-addon-release` workflow
+6. *Only for changes in the `config/splunk_matrix.conf`: Follow the instructions from [Runbook to creating and publishing docker images used in reusable workflow](https://github.com/splunk/addonfactory-workflow-addon-release/blob/main/runbooks/addonfactory-workflow-addon-release-docker-images.md#runbook-to-publish-multiple-images-of-different-linux-flavors-and-versions-for-scripted-inputs-tests) to create and publish Splunk images for scripted inputs tests based on the updates in the matrix coniguration.