Add contributing task guide

Author: dnestoro

CONTRIBUTING.md

@@ -21,6 +21,24 @@ Then, point to the local repository in the configuration of either
 
 ## Contribute Metadata
 
+The suggested way to contribute to Reachability metadata repository is by using the `contributing` task.
+Before you start the task, you should have your tests implemented somewhere locally on your machine (please verify that tests are working on Java).
+
+In order to start this task, run (we suggest using `--console=plain` to reduce amount of gradle logging):
+
+```shell
+./gradlew contribute --console=plain
+```
+
+When started, the task will ask you few simple questions (like: where are your tests implemented, do your tests need resources, do your tests need docker images...). 
+In case you don't understand the question, just type "help".
+
+After it collects your answers, the task will:
+- generate necessary infrastructure
+- copy your tests to the proper location
+- generate metadata and store it in the proper location
+- ask you if you want to create a pull request, or you want to keep working on it locally
+
 ### Checklist
 In order to ensure that all contributions follow the same standards of quality we have devised a following list of requirements for each new added library.
 `org.example:library` project is also included as a template for new libraries.