Update README.

README.md

@@ -12,13 +12,45 @@ Starting from a list of git projects and a Bash fragment, for each project, this
 * [Pipeline: Shared Groovy Libraries](https://plugins.jenkins.io/workflow-cps-global-lib) ver. 2.15 or newer
 * User credentials configured on Jenkins. These are needed to set the author of the tag.
 
+## Parameters
+* _Git Root_: The URL of the parent organization that includes all the listed repositories.
+* _List of Repositories_: A plain text file, one line for each Git repository name to update.
+* _Action Root_: The root URL of the Bash fragment to execute.
+* _Action File_: The relative path under the Action Root of a Bash script 
+
+## How Does It Work
+The pipeline:
+1. clones one repository at the time
+2. downloads the file _Action Root_/_Action File_ in the root folder of the repository
+3. runs the action file using the root folder of the repository as working dir
+4. pushes the repository back to Gitea
+
+Every change to the Git repository **MUST** be done by the action file. The pipeline is in charge only
+to push the repository to the remote origin.
+
 ## Sample Usage
 
 ### List of Repositories
+````text
+52n-wps-algorithm-gcube
+52n-wps-server-gcube 
+about-vre 
+accept-invite-portlet 
+accounting-aggregator-se-plugin 
+accounting-analytics 
+...
+````
 
-### Bash Fragments
+### Action File
+The following fragment downloads a file named LICENSE.md and adds it to the repository:
+
+````bash
+#!/usr/bin/env bash
+curl <my URL>/LICENSE.md -o LICENSE.md
+git add LICENSE.md
+git commit -m "LICENSE.md"
+````  
 
-TBP
 
 ## References
 * [Pipeline as code](https://jenkins.io/doc/book/pipeline-as-code/)