added IssueTest docs to contribution guidelines

Author: erosb

CONTRIBUTING.md

@@ -4,7 +4,37 @@ Contributing guidelines
 Submitting bugreports
 ---------------------
 
+Though any kind of feedback, feature request and bugreport is appreciated, I would more prefer if you would send a proper
+pull request with a failing test in case you find a bug in the library.
 
+Currently there is a simple mechanism in the integration tests to make it easy to add new tests. Using this is especially
+preferred if your bugreport is related to a complex schema or multiple schemas. There is no need to dig into the java code,
+you only have to create a few new files in the repo (but you will have to run the tests - see the build instructions below).
+
+Steps:
+ * create an issue, just to get an issue number
+ * fork the repository
+ * in your fork, create a directory under the `tests/src/test/resources/org/everit/json/schema/issues/` directory (for example `issue42` )
+ * in this directory create a `schema.json` file with your JSON Schema document that is not handled correctly
+ * in the same directory create a `subject-valid.json` file, which is a JSON document, and you expect that document to pass
+the validation, but due to a bug it fails with a `ValidationException`
+ * if you have a JSON document that you expect to be invalid, but it passes the validation, then you should name this file `subject-invalid.json`.
+It will mean that for the test suite that an expected ValidationException is not thrown.
+ * you can create both the `subject-valid.json` and `subject-invalid.json` test files if you find it needed
+
+
+Remote schema loading:
+If your testcase has anything to do with remote schemas, then
+ * you can put those schemas under the `yourIssueDir/remotes/` directory
+ * the `yourIssueDir/remotes/` directory will act as the document root of a HTTP server during test execution
+ * this HTTP server will listen at address `http://localhost:1234` so please change your schemas (`id` and `$ref` properties)
+to fetch the remote schemas relative from this address
+
+You can find a good example for all of these in the `tests/src/test/resources/org/everit/json/schema/issues/issue17` testcase.
+
+If you successfully created your testcase, then it will fail with an `AssertionError` with a message like
+"validation failed with: org.everit.json.schema.ValidationException:..." or "did not throw ValidationException for invalid subject",
+and then you are ready to send a pull request.
 
 
 Building the project locally
@@ -18,3 +48,5 @@ Prerequisities: the following tools have to be installed:
 Steps for building the project:
 * clone the repository: `git clone https://github.com/everit-org/json-schema.git && cd json-schema/`
 * build it with maven: `mvn clean install`
+
+(or just `mvn clean test` if you are only interested in running the tests)