Update readme

README.md

@@ -15,3 +15,8 @@ If you are going to create a new test case, please follow the structure inside t
 
 _Important note: MIME normaliser does not cover all provided test cases.
 Some of them explicitly ignored to indicate which tests has to be fixed. Please refer to `failingCases` arrays inside the test files._
+
+Tests are using both JSDOM and custom Vereign [DOM parser](https://code.vereign.com/code/js-toolbox/gsdom).
+Vereign DOM parser has been developed to support MIME normalisation in Google Add-on and is a must for testing.
+
+Whenever you develop a new normalisation logic, ensure that Vereign DOM parser support functions you apply.