Added brief docs about adding tests for issues (#36)

Author: Nyholm

Readme.md

@@ -42,3 +42,16 @@ $finder->in('/foo/bar');
 //Start extractring files
 $sourceCollection = $extractor->extract($finder);
 ```
+
+## Found an issue?
+
+Is it something we do not extract? Please add it as a tests. Add a new file with your example code in
+`tests/Resources/Github/Issue_XX.php` then edit the `AllExtracotrsTest` to make sure the translation
+key is found. 
+
+```php
+
+// ...
+$this->translationExists($sc, 'trans.issue_xx');
+
+```