[incubator-kie-issues#1023] Add Readme from PR suggestion

kie-dmn/kie-dmn-test-resources/Readme.md

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+DMN Test Resources
+==================
+
+This module is meant to be the ultimate single-source-of truth about DMN models used for testing purposes.
+
+The models are stored under `src/test/resources`, so the module has to be imported has
+
+```xml
+<dependency>
+      <groupId>org.kie</groupId>
+      <artifactId>kie-dmn-test-resources</artifactId>
+      <classifier>tests</classifier>
+      <scope>test</scope>
+    </dependency>
+```
+
+to have them available.
+
+Models are split in two categories: _valid_models_ and __invalid_models_. 
+The former are valid models, expected to succeed the DMN validation.
+The latter are invalid ones, expected to have some errors, and used to check that such errors are detected by validation.
+
+For both categories, there is a subdivision: _DMNv1_x_ and _DMNV1_5_. 
+The former are all the models created before the 1.5 implementation, that are hard to sort based on the version relates to. 
+The latter contains models with 1.5. specific feature.
+In the future, for each new DMN release there will be a specific folder.