added contributing guide

Author: Chad Huff

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contribution
+
+In order to contribute to the improvement of the project, below you will find the project development flow.
+Before being able to open a PR with changes to be applied or bug fixes to be solved, it is necessary to open an issue 
+and explain the reason and a possible solution. You can open each PR by forking the project and creating the PR directly from 
+your project to the project of our library.
+
+Main rules:
+- Open issue
+- Create PR from forked project
+- Link your PR to the issue
+
+### Run tests
+
+```sh
+mvn clean test
+```
+
+### Build
+
+```sh
+mvn clean install
+```

README.md

@@ -260,3 +260,6 @@ CmpList cmpList = loader.cmpList(cmpListContent);
 |usct|12|GpcSegmentIncluded|Boolean. Default value is true|
 |usct|12|Gpc|Boolean|
 
+## Contributing
+
+Here you can find the [contributing guide](CONTRIBUTING.md) to help maintain and update the library.