Github CI: README badge.

README.md

@@ -7,3 +7,7 @@ The **Tester** module is a self explanatory example:
 - [`tester/pom.xml`](tester/pom.xml): the plugin example configuration.
 - [`tester/src/main/resources/tester-resource.txt`](tester/src/main/resources/tester-resource.txt): the resource template to be merged with external files.
 - [`tester/src/main/assets/`](tester/src/main/assets): the external files folder.
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+![Maven CI](https://github.com/antonio-petricca/filemerger-maven-plugin/workflows/maven-ci/badge.svg)